Infectious diseases, as you will gather, are curious things. Some flit about like Akureyri disease, popping up seemingly at random, then going quiet for a time before popping up somewhere else. Others advance across landscapes like a conquering army. West Nile virus surfaced in New York in 1999 and within four years had covered the whole of America. Some diseases wreak havoc and then quietly withdraw, sometimes for years, occasionally forever. Between 1485 and 1551, Britain was repeatedly ravaged by a terrifying malady called the sweating sickness, which killed untold thousands. Then it abruptly stopped and was never seen there again. Two hundred years later, a very similar illness appeared in France, where it was called the Picardy sweats. Then it too vanished. We have no idea where and how it incubated, why it disappeared when it did, or where it might be now.
Baffling outbreaks, particularly small ones, are more common than you might think. Every year in the United States about six people, preponderantly in northern Minnesota, grow ill with Powassan virus. Some victims suffer only mild flu-like symptoms, but others are left with permanent neurological damage. About 10 percent die. There is no cure or treatment. In Wisconsin in the winter of 2015–16, fifty-four people, from twelve different counties, fell ill from a little-known bacterial infection called Elizabethkingia. Fifteen of the victims died. Elizabethkingia is a common soil microbe, but it only rarely infects people. Why it suddenly became rampant across a wide area of the state, and then stopped, is anyone’s guess. Tularemia, an infectious disease spread by ticks, kills 150 or so people a year in America, but with unaccountable variability. In the eleven years from 2006 to 2016, it killed 232 people in Arkansas, but only one person in neighboring Alabama despite abundant similarities in climate, ground cover, and tick populations. The list goes on and on.
Perhaps no case has been harder to explain than Bourbon virus, named for the county in Kansas where it first appeared in 2014. In the spring of that year, John Seested, a healthy, middle-aged man from Fort Scott, about ninety miles south of Kansas City, was working on his property when he noticed he had been bitten by a tick. After a while he began to grow achy and feverish. When his symptoms didn’t improve, he was admitted to a local hospital and given doxycycline, a drug for tick-bite infections, but it had no effect. Over the next day or two, Seested’s condition steadily worsened. Then his organs began to fail. On the eleventh day he died.
Bourbon virus, as it became known, represented a whole new class of virus. It came from a group called thogotoviruses, which are endemic to regions of Africa, Asia, and eastern Europe, but this particular strain was entirely novel. Why it appeared suddenly in the very middle of the United States is a mystery. No one else got the disease in Fort Scott or anywhere else in Kansas, but a year later a man 250 miles away in Oklahoma came down with it. At least five other cases have since been reported. The Centers for Disease Control is curiously reticent about numbers. It says only that “as of June 2018, a limited number of Bourbon virus disease cases have been identified in the Midwest and southern United States,” a somewhat odd way of putting it because there is clearly no limit on the number of infections any disease can cause. The most recent confirmed case, at the time of writing, was a fifty-eight-year-old woman who was bitten by a tick while working in Meramec State Park in eastern Missouri and died soon afterward.
It may be that all of these elusive diseases infect lots more people, but not seriously enough to be noticed. “Unless doctors are doing laboratory tests specifically for this infection, they’ll miss it,” a CDC scientist told a reporter for National Public Radio in 2015, in reference to Heartland virus, yet another mysterious pathogen. (There really are a lot of these.) As of late 2018, the Heartland virus had infected some twenty people and killed an unknown number since it first appeared near St. Joseph, Missouri, in 2009. But so far all that can be said for sure is that these diseases only infect a very unlucky few people far removed from each other with no known connections between them.
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Sometimes it turns out that what seems to be a new disease is not new at all. Such proved to be the case in 1976 when delegates to an American Legion convention at the Bellevue-Stratford Hotel in Philadelphia began to fall ill from a disease no authority could identify. Soon many of them were dying. Within a few days, 34 were dead and another 190 or so were ill, some gravely. An additional puzzle was that about one-fifth of the victims had not set foot in the hotel, but had only walked past it. Epidemiologists from the Centers for Disease Control took two years to identify the culprit, a novel bacterium from a genus they called Legionella. It had spread through the hotel’s air-conditioning ducts. The unlucky passersby had been infected by walking through exhaust fumes.
Only much later was it realized that Legionella was almost certainly responsible for similarly unexplained outbreaks in Washington, D.C., in 1965 and in Pontiac, Michigan, three years later. Indeed, it turned out that the Bellevue-Stratford Hotel had suffered a smaller, less lethal cluster of pneumonia cases two years earlier during a convention of the Independent Order of Odd Fellows, but that had attracted little attention because no one died. We now know that Legionella is widely distributed in soil and freshwater, and Legionnaires’ disease has become more common than most people suppose. A dozen or so outbreaks are reported each year in America, and about eighteen thousand people become sick enough to need hospitalization, but the CDC thinks that that number is probably underreported.
Much the same thing happened with Akureyri disease where further investigations showed that there had been similar outbreaks in Switzerland in 1937 and 1939 and probably in Los Angeles in 1934 (where it was taken to be a mild form of poliomyelitis). Where, if anywhere, it was before that is unknown.
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Whether or not a disease becomes epidemic is dependent on four factors: how lethal it is, how good it is at finding new victims, how easy or difficult it is to contain, and how susceptible it is to vaccines. Most really scary diseases are not actually very good at all four; in fact, the qualities that make them scary often render them ineffective at spreading. Ebola, for instance, is so terrifying that people in the area of infection flee before it, doing everything in their powers to escape exposure. In addition, it incapacitates its victims swiftly, so most are removed from circulation before they can spread the disease widely anyway. Ebola is almost ludicrously infectious—a single droplet of blood no bigger than this o may contain a hundred million Ebola particles, every one of them as lethal as a hand grenade—but it is held back by its clumsiness at spreading.
A successful virus is one that doesn’t kill too well and can circulate widely. That’s what makes flu such a perennial threat. A typical flu renders its victims infectious for about a day before they get symptoms and for about a week after they recover, which turns every victim into a vector. The great Spanish flu of 1918 racked up a global death toll of tens of millions—some estimates put it as high as a hundred million—not by being especially lethal but by being persistent and highly transmissible. It killed only about 2.5 percent of victims, it is thought. Ebola would be more effective—and in the long run more dangerous—if it mutated a milder version that didn’t strike such panic into communities and made it easier for victims to mingle with unsuspecting others.