Coronavirus and Sunlight

kw: medicine, viruses, coronavirus, ultraviolet, inactivation

I contend that the safest place to see others face-to-face is outside on a sunny, breezy day. Masks are not needed outdoors, only indoors. Here is why.

If someone is ill with the novel coronavirus SARS-Cov-2, the cause of Covid-19 disease, any droplets that leave their body with their breath will contain virus particles, called virions on medical literature. The virions are rather large, some 125 nm in diameter. That's about four times the size of rhinoviruses, the most common cause of the common cold, and a it's a little larger than influenza viruses. However, 125 nm is 1/8,000th of a millimeter. No mask you can afford will capture them, but that is not the point.

Indoors among "the public", the threat is not isolated virions but the droplets released in small amounts by speaking or heavy breathing, in larger amounts by a cough, and in very great amounts by a sneeze. The droplets are mostly between 1,000 and 10,000 times the size of the virions they might contain. Nearly any such droplets that encounter your face covering or mask will be caught in it. So when you wear a mask to the store, when you get home wash it with hand soap and let it dry, or replace it if you have a lot of the disposable kind.

What happens outside? Unless the humidity is very high, the droplets evaporate in a few minutes, or even a few seconds. The virions are now "free". Even before the droplets evaporate, however, if the sun is up, something wonderful happens! Ultraviolet light from the sun destroys the virus genome. The question is, how soon?

I found and downloaded an article, "Predicted Inactivation of Viruses of Relevance to Biodefense by Solar Radiation", by C. David Lytle and Jose-Luis Sagripanti, in Journal of Virology, v79, No. 22, Nov. 2005, p. 14244–14252. The work was supported by DoD and relates to defense against biowarfare agents.

The authors gathered data on solar UV and Hg-vapor UV (such as that used in hospital disinfection lamps), and how quickly different kinds of viruses are destroyed. There are a lot of details, but the relevant conclusions related to coronaviruses are these:

• Sensitivity units are reported two ways, as D37 and as 1-log.
• D37 is the dose of UV needed to destroy all but 37% of the particles. 0.37 is 1/e, and is related to natural logarithms.
• 1-log is the dose of UV needed to destroy 90% (all but 10%) of the particles. It is related to common logarithms, of base 10. This is more understandable and relevant to us.
• The viruses of greatest interest in the article are filoviruses such as Ebola or Marburg. D37 for these is about 7.4 and 1-log is 17. (These data are from Tables 2 and 4)
• The virus of interest today is a coronavirus; the family Coronaviridae has D37 in the range 2.5-3.9 (Table 2), for which I calculate 1-log of 5.7-9.0. Thus SARS-Cov-2 is about twice as sensitive to UV light as the Ebola virus. That's good news.
• The effectiveness of solar radiation depends on the angle of the sun. A specific datum relevant to the DE-PA border area where I live is for Davis, CA, on a typical, sunny July 15. The 1-log level of deactivation for a filovirus occurs in 55 minutes. (If you're lucky enough to live in Hawaii, the sun's UV is twice as strong there, and the relevant time is 21 minutes.)

From these data I calculate that 1-log deactivation (10% remaining viable) takes between 19 and 29 minutes. Let's round these to 20 and 30 minutes. Deactivation is a stochastic matter. If a UV photon passes through the right part of a coronavirus virion, it will damage the RNA. Most of them miss. So let's look at the 20-minute level for a theoretical virus. In 20 minutes, 10% are still viable. In 40 minutes, only 1% are still viable. In an hour, 0.1% are still viable, and so it goes. That is one in a thousand after an hour, and one in a million after two hours, and one in a billion after three hours. For a "tougher" coronavirus, at the 30-minute end of that range, viability after an hour is about 1%, and after three hours it is about one in a million.

So if you're worried that someone emitting viruses is outside, and they blow downwind, two things work in your favor. Firstly, the swirling wind will spread them out so only a few have the chance to reach you, and secondly, if the person is some distance "upwind" from you, sunlight will zap the virions at a steady rate.

There are two further matters I am still researching. 

• How many virions need to enter the body to cause infection? A very infectious virus such as influenza can stably cause infection if around 100 virions are breathed in. This is because many of them are "beaten" by the innate immune system. For more susceptible people, not as many are needed, but I don't know what the normal range of natural resistance is, for people who are not vaccinated. It may not yet be known what the "average infectious load" (my term) is for coronavirus.
• Whether it is sunny or not, oxygen also deactivates virions. How quickly? Whatever I can find out, I'll report.

The worst virus

kw: book reviews, nonfiction, medicine, viruses

What ancient disease is 100% fatal without treatment? For that matter, even more "modern" diseases such as Ebola and Marburg are not 100% fatal. This image shows the face of the most-feared disease in history.

These look like hollow-point bullets, but they are far worse. This is the rabies virus. It is rare enough in the First World that many doctors have never seen a case of human rabies. In the U.S., about 3 cases (fatalities) occur yearly, in spite of the existence of an effective treatment. However, worldwide, the death toll is about 55,000. Sad to say, that still makes it a "rare" disease, and in many countries there is little incentive to spend money to vaccinate all the dogs, or keep on hand sufficient amounts of the post-exposure vaccine to treat more than a fraction of the human cases.

The husband-wife team of Bill Wasik and Monica Murphy have written Rabid: A Cultural History of the World's Most Diabolical Virus. While there is medical information aplenty, the emphasis is on the effect of Rabies and other zoonoses on human society during at least the past 4,000 years. Rabies not only has a folklore all its own, the peculiar way the disease progresses has also spawned two major cultural threads: vampires and werewolves. I'll leave the details to your imagination.

The folk horrors are not based on human cases of Rabies, but on observations of dogs. The Mad Dog phenomenon is well known. Humans infected with the virus, once it reaches the brain, exhibit hydrophobia, but very rarely bite or attack others. Also, the saliva of human cases contains little of the virus. Humans are dead-end hosts. The two hosts most responsible for spreading the disease are dogs and bats, although the virus can infect any mammal, and any mammal can catch it from any infected mammal, although I suspect a mouse or shrew would have a hard time piercing the skin of an elephant or rhino and infecting it.

The course of infection is unusual, and fortuitously led to a way to produce a post-exposure vaccine for humans, developed by Louis Pasteur in 1880-85. A bite, whether by dog, bat, raccoon or skunk (the most common hosts), deposits virus-laden saliva in the wound. Immediate cauterization is known to destroy the contagion, and this is the only historical remedy that has any chance of working. It is still used in the Third World. Within a day or so, however, the virus particles have found their way to a nerve and are making their way toward the brain. They travel at a rate of about 2 cm per day, so a bite on the hand or food could be dealt with by amputation, but this seems to be very rare. Once the viruses reach the brain, death is certain, in an average of four days.

The Milwaukee Protocol, developed by Dr. Rodney Willoughby in 2004, offers only a smidgen of hope. That is the year a girl survived brain infection with Rabies, while a medically induced coma and intensive supportive therapy kept her alive as her body developed an immune response strong enough to drive out the virus. Since that time about 8% of patients given this therapy have survived. A 92% death rate is marginally better than a 100% death rate.

The Pasteur vaccine and more recent (and less painful) vaccines develop the body's immune response during the time of nerve transmission, preventing the virus from reaching the brain. These vaccines, and the preventive vaccine given to most dogs in the West, are responsible for the very low Rabies death rate here. If 70% of dogs worldwide could be vaccinated, and human vaccines were made globally available, the world incidence of Rabies death would be about 70. To reduce it further would require some kind of vaccination program for raccoons and skunks and bats. How would you vaccinate billions of bats?

Rabies is but one of many zoonoses, or human diseases that originated in animals. I suspect all human diseases actually began as zoonoses. The ones most anciently associated with us began when we were another species! They evolved along with us. The most familiar zoonosis is influenza. Many kinds of flu virus circulate among birds and swine, in particular, but also among other animals. Most years, the Northern winter season leads to a sweep of flu across the globe and claims as many as half a million lives worldwide. The 1918 "Spanish flu" (that originated in swine) was unusually virulent and killed 40 million worldwide. For a sense of scale, malaria kills between 700,000 and 1.2 million yearly, depending on which "authority" you believe. The most famous recent zoonosis is AIDS, which seems to have jumped the ape-human barrier several times in the early 1900s, and now kills about 2 million yearly, having outstripped malaria since the 1980s.

In the book's closing chapter, the authors introduce an interesting twist. Knowing that the Rabies virus crosses the blood-brain barrier, how does it do it? Diseases such as meningitis, once they get into the brain, are usually fatal, and brain infections in general are intractable because none of our antibiotics can get into the brain except by direct injection. Who wants to have a hole bored in their skull? (I do, if it will save my life!). Pieces of the Rabies virus envelope can apparently hitchhike into the brain on "trusted" proteins. Thus this most fearsome of diseases may soon be harnessed to help us conquer other brain infections.

Are all viruses pathological?

kw: viruses, medicine

In a recent article in Wired (found here at wired.com), the question is raised, if we develop broad-spectrum antiviral medications, should we use them? Since the publication of that article just a month ago, about sixty online articles have explored the idea. You can find them and other similar articles by searching for "beneficial viruses" (include the quotes for a phrase search).

Are all viruses bad? Do they all cause disease?

When bacteria were first discovered, it was thought by many that all "germs" were bad, and once antibiotics began to be developed in the 1930s, they were used indiscriminately for any sign of infection. Such things as upset digestions, diarrhea and bloating were thought to be unfortunate side effects. But it didn't take long for our "internal flora" to be discovered, and we are still learning how important they are. The list of beneficial bacteria, some residing in our gut, some on our skin, some lining our sinuses, and others who knows where, continues to grow. By current estimates, 90% of the cells in a "human" body are bacterial, though they make up no more than 2% by weight.

It is no more than a decade or two since it was discovered that in a typical sample of ocean water, there are a thousand virus particles ("virions") for every eukaryotic cell, most of them being bacteriophages. Or, I should say, denizens of bacteria, because it is not known whether a virus residing in a bacterial cell is there to kill it or in some way to help it. We know that phages are pathological to bacteria, just as many viruses that infect us and our animals and plants are pathological. But we are just beginning to learn of viruses that are found in cells yet don't seem to cause disease.

Wouldn't it be ironical if we developed a broad-spectrum antiviral, tried it out, and found it to be universally fatal to the mouse, monkey or man into which it was introduced? Fatal why? Because it eliminated a virus to which we play host, that performs a required function! Fortunately, while our internal flora of bacteria may be helpful, none is required for us to continue living. Strains of supposedly germ-free mice have been developed, and though they live differently than ordinary mice—mainly in that they need to eat more—they seem to live well enough. But we don't even know if those mice are virus-free. We don't yet know how to produce a virus-free mouse, or if it is possible to do so.

The days of effective antibiotic medicines are drawing to a close. We are being forced to take another look at an older, effective, if cumbersome, therapy using bacteriophages. The problem is, these are very specific. There are no broad-spectrum phages. A second kind of therapy (this is very early days) is the anti-bacterial bacterium: using an overwhelming dose of our good bacterial companions to drive out those we don't want.

This makes me wonder, are there anti-viral viruses? Are we actually host to any (or many?) viruses, not yet discovered or studied, that keep pathological viruses in check most of the time? Just as certain bacterial are becoming known as essential ingredients in our immune function, there may also be immune-functional viruses.

This just scratches the surface of the questions we need to be asking about viruses and our relationship to them. There is a long way to go, and we ought to be careful how we use new "miracle" drugs, lest the miracle we perform is to our own detriment.

The most primitive life is still with us

kw: book reviews, nonfiction, viruses

I have read a number of books on viruses and virology, including a few that I have reviewed in this blog. The latest covers no new ground, but is a very informative introduction to the modern view of viruses: A Planet of Viruses by Carl Zimmer.

Using a baker's dozen case accounts to cover the breadth of the subject, Zimmer introduces us to viruses large and small, from ancient foes to recent eruptions. Though it may have been with us the longest, smallpox was the first to be cured, partly because it is the most obvious. In contrast to HIV, which is very recent, smallpox makes a person sick immediately, with unmistakable symptoms, and runs its course, deadly or not, in a few weeks. This has led to it being the first virus to be eradicated in the wild.

Viruses are fearsome in part because there are no known "beneficial" varieties. Just as snakes are universally predatory, viruses thrive only by parasitism of cellular organisms. As it happens, though, just as there are billions of bacteria for every "higher" organism, there are huge numbers of virus varieties that parasitize only bacteria and are thus beneficial to us. Before the discovery of antibiotics, viruses called bacteriophages (for "eaters of bacteria") were cultivated and used to cure bacterial diseases. Their only drawback is that viruses are very specific, so it takes quite a cocktail of phages is needed to combat bacteria that exist in multiple strains.

It is now known that the sea is a "virus ocean", with many millions of virus particles per liter of sea water. It is likely that, without viruses, the seas would become a cesspool of bacterial goop! The air is filled with suspended viruses as well, though to a lower density. As numerous as they are, viruses are so small that it takes a few million to outweigh the average bacterium, so they are (probably) not the heaviest component of the biosphere.

A recent discovery shows they are not all that small. Mimiviruses are called that because they mimic small bacteria. They are visible in an optical microscope, being about a micron in size. The smallest viruses known are one-hundredth the size, and most are about one-fortieth to one-twentieth that size.

The most interesting viruses to me are the retroviruses, those that insert their genomes within the genome of their host. So many of these have become "endogenous", meaning incorporated permanently, that about 8% of any animal's genome, including ours, consists of viruses that can be reactivated (according to other accounts I have read, about another quarter of our genome consists of fragmentary virus genomes).

If we consider the ways that life may have originated, it is likely that viruses may have either preceded the earliest cells, or that they arose along with them. That means that living things have never existed in isolation, but have always partaken of a grand kind of cross-species interbreeding facilitated by viruses. They are sometimes called the third sex, although before binary sex arose, they'd have been called the opposite sex! (were there anybody there with sufficient brains to do the calling).

The book is an easy read, and an enjoyable one. For many, it will introduce many subjects that one can then pursue in other works, and the bibliography contains plenty of excellent resources for that.

Our most intimate inmates

kw: book reviews, nonfiction, viruses, evolution

Either physician and researcher Frank Ryan and the colleagues he interviewed while writing Virolution are on the brilliant forefront of evolutionary discoveries, or they are stark, staring mad. I tend to believe the former is true. But consider what the book claims:

• Viruses are primarily beneficial, even necessary for our existence, and that of all life.
• Viruses that reside in our genome mediate the development of our bodily organs.
• While "vertebrate DNA" makes up 1.5% of the total human genome, ready-to-activate viruses make up 6-7%, and partial viruses make up another 35-40% (more are being determined all the time).
• These partial viruses are experts at moving about, and such "jumping genes" are huge agents of genetic change, greatly increasing the variation that natural selection depends upon to produce new species.
• Viral symbiosis and mutualism are responsible for our continued health and longevity.
• Virus infections and diseases are an unfortunate side effect of the ages-long interplay of viral and animal (and plant) DNA.

I don't know about you, but I find some of these ideas rather unsettling. I remember predicting, nearly twenty years ago, that within a few generations, ever person still living on Earth would be either immune to or tolerant of HIV infection. It seems Dr. Ryan would agree, and moreover, he would further predict that HIV-1 and -2 would become integrated into our genome, as at least 98,000 other retroviruses have done in past ages, going back to the first cells. And that figure is actually a tip-of-the-iceberg amount; each virus has infected numerous times. The number of complete virus genomes and genome fragments in the DNA of every creature is several million.

Well, I hope I got all that correct. The book is fascinating reading. It points to HIV and to a virus that is currently decimating koalas as examples of early stages in the integration of a new virus into the genome. This ruthless culling made me recall something else I read in an article many years ago.

I don't recall author or title, but the premise was this: Viruses descend from a toolbox of small, initially non-living DNA "machines", created to facilitate regulation of DNA in multi-cellular creatures. They became the original "Frankenstein monsters", having attained great powers to modify DNA and create copies of themselves. Achieving a kind of quasi-life, they did what life does, and began to reproduce selfishly. They have become the prototype of the "gray goo" that some researchers fear will result if we produce self-reproducing nanomachines. The fact that we have not become a world of gray goo, AKA virus fodder, is that, in self-defense, early control mechanisms evolved just quickly enough into a more robust and active immune system. This virus-versus-immune system arms race has now gone on for about two billion years.

Whichever way viruses arose, Frank Ryan's claim is that they are primarily symbiotic with us and with all plants, animals and fungi. In a late chapter in the book, he outlines epigenetics, the subject of a book I reviewed two weeks ago. I saw no obvious connection between epigenetics and virology, but if I understood right, the various mechanisms of DNA control that we lump under epigenetics also activate and deactivate retroviral genomes that are so intimately involved in our development from a fertilized ovum to a grown adult, and throughout our lives.

Dr. Ryan's interest is not only academic. He is a physician, with a doctor's practicality. Pathological cases and other problems help researchers figure out the difference between things working right and working wrong, or not working at all. Diseases highlight areas that need to be understood. Pathologies that were once thought to be this or that "bad gene" are now often shown to be problems of development, or or epigenetic mistakes, or of a DNA-virus interaction gone wrong. Once they are better understood, therapies that attack the proper cause can be developed. Because epigenetics is so variable from person to person, because of our differing experiences, this will inevitably lead to very personalized medicine, almost the way my eyeglasses will only work with my eyes, and you need your own pair with different parameters, if you need any at all. If future DNA-HERV-epigenetic medicine can be done at acceptable cost, the possibilities are breathtaking.

The book is written at a bit higher level than many popularizations, but I didn't find the reading itself to be difficult. The concepts, however, are so mind-blowing that I'll have to set the book aside a while and re-read it later to be sure my impressions are in any way accurate.

Viruses R Us

kw: medicine, viruses, embryology, symbiosis

I'll just get this out of the way before I even finish the book. I find the idea behind this image rather unsettling. This shows a portion of the syncytium (pronounced sin-sigh-tee-um), the multinuclear membrane, effectively composed of a single cell, that is the boundary between fetal blood and maternal blood in the placenta of all mammals. It is the reddish layer surrounding the purplish blobs, which are folds of placental tissue.

What you don't see in this light microscope image are the viruses that induce the syncytium to form. Animal tissue doesn't "know" how to form a syncytium, or any multinuclear cell. Its formation is mediated by viruses called HERVs, for Human Endogenous RetroViruses. The retrovirus most of us have heard about is HIV, the cause of AIDS. It is related to HERVs. Our DNA is host to many, many related retrovirus genomes, and certain ones are expressed and work together with "our" DNA at many stages of our life, including setting up the placenta that makes most mammalian pregnancies work.

I'm reading a book on evolutionary virology, which I'll review more fully in a few days. Meantime, I could not get this image out of my mind once I saw it at the author's website. Many of our tissues and organs develop with the help of symbiotic viruses. I never knew viruses could be symbiotic! Not only that, they may be the dominant partner!!

This is one more demotion of our vaunted humanity. First, we were at the center of the Universe. Copernicus and Galileo moved Earth to "third rock from the Sun". Then, we were the peak of creation. Darwin, Wallace and others showed we're smart apes, but apes all the same. In recent decades it has become clear that 9/10 of the living cells in our bodies are bacteria, although each of "our" cells weighs hundreds of times what a bacterium does. Now I read that, while "vertebrate DNA" makes up only 1.5% of our total genome, various total and partial virus sequences make up 45%, or 30 times as much. I am starting to think that if you took away everything that is not "human" from us, we would be nearly weightless shells, ready to collapse under our own negligible mass.

I'm tempted to write more about the book now, but I suspect the author has more surprises waiting in the last few chapters, so I must simply say, "Stay tuned."

A Narrow, narrow miss

kw: book reviews, nonfiction, viruses, biology

The Colonels Jaax, Nancy and Jerry. In 1983, Nancy Jaax came within a few microns (the thickness of a surgical glove) of being infected with Ebola Zaire, the "hottest" strain of Ebola virus. It was her second time working in a BSL 4 containment, in a "blue suit", and her first experience doing any work there.

BSL 4, BioSafety Level 4, describes a total isolation (as near total as is technically possible) between specimens containing infectious viruses or bacteria, and those who work with them. This very costly sort of "clean room" (or very, very dirty room, if you think of it) containment for specimens, including living (temporarily) lab animals such as infected monkeys, is reserved for a handful of the most dangerous agents, including several strains of Ebola and the Marburg virus.

These two relates species make up the genus of the filoviruses, or thread viruses. They and other BSL 4-rated species kill 10% or more of their victims, typically within a week or two of exposure. By contrast, the much-feared AIDS virus, HIV, though it seems to kill at least half, takes one or two decades to do so.

Have you ever had surgery, with the antiseptic washes, the surgeons and nurses in scrubs, masked and gloved? If so, you were in a BSL 2 containment, and it was you who were being protected. It costs tens of thousands of dollars (or Euros) to set up BSL 2 facilities, and tens of millions for BSL 3 or 4.

This is the Marburg Virus. Ebola looks about the same. It takes an expert to distinguish the various thread viruses from photographs. Biochemical and DNA tests are needed to be sure which strain and which species is which.

In a way, these are very primitive viruses. The thread is a protein structure about 70nm in diameter, or about 1/20th the diameter of an E. coli bacterium. The structure is hollow, containing a simple coil of RNA and another protein or two that initiate RNA transcription and duplication once the virus is taken into a cell.

Cells can be remarkably stupid. They tend to take anything into themselves that is coated with protein. Many viruses take advantage of this.

Once a cell contains an active ("living" is not really the right word) virus particle, it changes completely. Enzymes in the cell help the virus remove its protein coat, and others get suborned by the RNA and replicase proteins. The cell begins to make copies of the virus's RNA. The raw RNA is a (-) strand. Copies from this strand are (+) strands. The (+) strands and the cell's ribosomes begin making virus proteins (there are fewer than ten for a filovirus).

Once some (+) strands have been made, the replicase proteins will, by chance, produce equal numbers of (+) and (-) strands of RNA. The (-) strands and the virus proteins self-assemble into new virus particles (the uncoating enzymes have been deactivated by this point). The (+) strands keep churning out new proteins. In the case of Ebola and Marburg, the cell gradually fills with one or more "bricks" of compacted virus particles. When a brick contacts the cell wall, the cell ruptures, spilling out a few million viruses that infect all the cells in the vicinity.

The above sequence takes no more than a few hours. Because filoviruses travel through the blood stream, they invade the whole body in short order. Cells of every kind, in every imaginable location throughout the body, become filled with virus bricks, then burst, and so on. Within a few days, to at most two weeks, symptoms such as headache begin. Depending on which specific strain one has, there is a chance between 24% and 90% that the whole body will melt down and turn to a mixture of destroyed cells and viruses. This kind of explosive amplification can result in an ounce or more of each pound of body weight becoming virus. That is whole continents, whole planets, whole galaxies full of viruses, billions of billions of billions, taking over a body in just those few days.

It is not known what species is the reservoir for any strain or species of filovirus. One of the very few things we think we know: All known filoviruses are very, very deadly to primates, including humans.

The above is good background to prepare your mind for reading The Hot Zone by Richard Preston. Just nineteen years ago, around Thanksgiving time in 1989, there was an outbreak of a new strain of Ebola in a "monkey house" in Reston, Virginia. Reston is a suburb right outside the Washington, D.C. beltway. The monkey house in question housed 500 monkeys from the Philippines, that were being prepared to be sent to laboratories around the country. These 500 never made it out of the building...

The Hot Zone chronicles the Reston outbreak, sandwiching it between a clear, chilling introduction to Marburg and the two main strains of Ebola, and an account of the author's pilgrimage in 1993 to Kitum Cave, in Kenya. The Marburg virus is thought to originate near there, perhaps in one of the species endemic to the cave...but we don't know for sure.

The book lacks an index. It lacks little else. It has been called a bio-thriller, but it is much more than that. The language is not hyped in the way you'd expect of a fictional thriller. It is as matter-of-fact as butter on bread, but as compelling as a ride on a roller coaster. Early on, I looked up some of the principal players, such as the Jaaxes shown above, especially because I couldn't bear the thought that she might have died.

She lived, and lives today. She and her husband were central to the military operation that (with the company's permission) cordoned off the monkey house, made the whole place into a BSL 4 containment, and destroyed all life within it, first the monkeys and the viruses they contained, then every insect, bacterium, and virus within its walls. That is, everything was killed that the team was able to detect and verify as dead. One can never know for sure.

Strangely, the Reston strain of Ebola is not known to have caused any human deaths...yet. We can't say if we dodged a bullet, or if the bullets simply were a strain that kills monkeys but doesn't affect humans. We simply don't know if there are lots of other filoviruses out there that can't infect people. We only know of the six or seven that can.

Author Preston makes it clear that there is no way to ensure that this won't happen again. The tropics are a day away by modern aircraft. A person infected with Ebola has three to ten days of no symptoms, in which to travel to London, New York, Paris, Tokyo, New Delhi or Beijing...and at least the Reston strain and one other can be transmitted in the air, by a cough or perhaps just by speaking. At any time we could be less than a month away from the near-depopulation of planet Earth.

Diseases caused by diseased bacteria

kw: illnesses, viruses, bacteria, pathogenesis

While researching another topic, I encoutered this article on Lysogeny, which refers to the life cycle of viruses that insert their genomes into host cells. Herpes is the best known example, with HIV a close second (how well known, not numbers infected).

I'd heard in the past that some diseases are caused by normally harmless bacteria being themselves infected by viruses. In some cases, the viruses carry the DNA for the disease toxin. These four lysogenic diseases are mentioned in the article:

• diphtheria
• cholera
• botulism
• scarlet fever

What a list!