As well as bacteria, your personal repertoire of microbes consists of fungi, viruses, protists (amoebas, algae, protozoa, and so on), and archaea, which for a long time were thought to be just more bacteria but actually represent a whole other branch of life. Archaea are very like bacteria in that they are quite simple and have no nucleus, but they have the great benefit to us that they cause no known diseases in humans. All they give us is a little gas, in the form of methane.
It’s worth bearing in mind that all these microbes have almost nothing in common in terms of their history and genetics. All that unites them is tininess. To all of them, you are not a person but a world—a vast and jouncing wealth of marvelously rich ecosystems with the convenience of mobility thrown in, along with the very helpful habits of sneezing, petting animals, and not always washing quite as fastidiously as you really ought to.
II
A VIRUS, IN the immortal words of the British Nobel laureate Peter Medawar, is “a piece of bad news wrapped up in a protein.” Actually, a lot of viruses are not bad news at all, at least not to humans. Viruses are a little weird, not quite living but by no means dead. Outside living cells, they are just inert things. They don’t eat or breathe or do much of anything. They have no means of locomotion. We must go out and collect them—off door handles or handshakes or drawn in with the air we breathe. They do not propel themselves; they hitchhike. Most of the time, they are as lifeless as a mote of dust, but put them into a living cell, and they will burst into animate existence and reproduce as furiously as any living thing.
Like bacteria, they are incredibly successful. The herpes virus has endured for hundreds of millions of years and infects all kinds of animals—even oysters. They are also terribly small—much smaller than bacteria and too small to be seen under conventional microscopes. If you blew one up to the size of a tennis ball, a human would be five hundred miles high. A bacterium on the same scale would be about the size of a beach ball.
In the modern sense of a very small microorganism, the term “virus” dates only from 1900, when a Dutch botanist, Martinus Beijerinck, found that the tobacco plants he was studying were susceptible to a mysterious infectious agent even smaller than bacteria. At first he called the mysterious agent contagium vivum fluidum but then changed it to “virus,” from a Latin word for “toxin.” Although he was the father of virology, the importance of his discovery wasn’t appreciated in his lifetime, so he was never honored with a Nobel Prize, as he really should have been.
It used to be thought that all viruses cause disease—hence the Peter Medawar quotation—but we now know that most viruses infect only bacterial cells and have no effect on us at all. Of the hundreds of thousands of viruses reasonably supposed to exist, just 586 species are known to infect mammals, and of these only 263 affect humans.
We know very little about most other, nonpathogenic viruses because only the ones that cause disease tend to get studied. In 1986, a student at the State University of New York at Stony Brook named Lita Proctor decided to look for viruses in seawater—which was considered a highly eccentric thing to do because it was universally assumed that the oceans have no viruses except perhaps for a transient few introduced through sewage outfall pipes and the like. So it was a slight astonishment when Proctor found that the average quart of seawater contains up to 100 billion viruses. More recently, Dana Willner, a biologist at San Diego State University, looked into the number of viruses found in healthy human lungs—somewhere else that viruses were not thought to lurk much. Willner found that the average person harbored 174 species of virus, 90 percent of which had never been seen before. Earth, we now know, is aswarm with viruses to a degree that until recently we barely suspected. According to the virologist Dorothy H. Crawford, ocean viruses alone if laid end to end would stretch for ten million light-years, a distance essentially beyond imagining.
Something else viruses do is bide their time. A most extraordinary example of that came in 2014 when a French team found a previously unknown virus, Pithovirus sibericum, in Siberia. Although it had been locked in permafrost for thirty thousand years, when injected into an amoeba, it sprang into action with the lustiness of youth. Luckily, P. sibericum proved not to infect humans, but who knows what else may be out there waiting to be uncovered? A rather more common manifestation of viral patience is seen in the varicella-zoster virus. This is the virus that gives you chicken pox when you are small, but then may sit inert in nerve cells for half a century or more before erupting in that horrid and painful indignity of old age known as shingles. It is usually described as a painful rash on the torso, but in fact shingles can pop up almost anywhere on the body surface. A friend of mine had it in his left eye and described it as the worst experience of his life. (The word, incidentally, has nothing to do with the tiles of a roof. Shingles as a medical condition comes from the Latin cingulus, meaning a kind of belt; as a roofing material, it is from the Latin scindula, meaning a stepped tile. It is just by chance that they ended up in English with the same spellings.)
The most regular of unwelcome viral encounters is the common cold. Everyone knows that if you get chilled, you are more likely to catch a cold (that is why we call it a cold, after all), yet science has never been able to prove why—or even, come to that, if that is actually so. Colds unquestionably are more frequent in winter than in summer, but that may only be because we spend more time indoors then and are more exposed to others’ leakages and exhalations.
The common cold is not a single illness but rather a family of symptoms generated by a multiplicity of viruses, of which the most pernicious are the rhinoviruses. These alone come in a hundred varieties. There are, in short, lots of ways to catch a cold, which is why you never develop enough immunity to stop catching them all.
For years, Britain operated a research facility called the Common Cold Unit, but it closed in 1989 without ever finding a cure. It did, however, conduct some interesting experiments. In one, a volunteer was fitted with a device that leaked a thin fluid at his nostrils at the same rate that a runny nose would. The volunteer then socialized with other volunteers, as if at a cocktail party. Unknown to any of them, the fluid contained a dye visible only under ultraviolet light. When that was switched on after they had been mingling for a while, the participants were astounded to discover that the dye was everywhere—on the hands, head, and upper body of every participant and on glasses, doorknobs, sofa cushions, bowls of nuts, you name it. The average adult touches his face sixteen times an hour, and each of those touches transferred the pretend pathogen from nose to snack bowl to innocent third party to doorknob to innocent fourth party and so on until pretty much everyone and everything bore a festive glow of imaginary snot. In a similar study at the University of Arizona, researchers infected the metal door handle to an office building and found it took only about four hours for the “virus” to spread through the entire building, infecting over half of employees and turning up on virtually every shared device like photocopiers and coffee machines. In the real world, such infestations can stay active for up to three days. Surprisingly, the least effective way to spread germs (according to yet another study) is kissing. It proved almost wholly ineffective among volunteers at the University of Wisconsin who had been successfully infected with cold virus. Sneezes and coughs weren’t much better. The only really reliable way to transfer cold germs is physically by touch.