That is, of course, no grounds for complacency. Ebola was only formally identified in the 1970s, and until recently all its outbreaks were isolated and short-lived, but in 2013 it spread to three countries—Guinea, Liberia, and Sierra Leone—where it infected twenty-eight thousand people and killed eleven thousand. That’s a big outbreak. On several occasions, thanks to air travel, it escaped to other countries, though fortunately in each instance it was contained. We may not always be so lucky. Hypervigilance makes it less likely diseases will spread, but it’s no guarantee that they won’t.
It’s remarkable that bad things don’t happen more often. According to one estimate reported by Ed Yong in The Atlantic, the number of viruses in birds and mammals that have the potential to leap the species barrier and infect us may be as high as 800,000. That is a lot of potential danger.*2
II
IT IS SOMETIMES said, only partly in jest, that the worst health initiative in history was the invention of agriculture. Jared Diamond has called it “a catastrophe from which we have never recovered.”
Perversely, farming didn’t bring improved diets but almost everywhere poorer ones. Focusing on a narrower range of staple foods meant most people suffered at least some dietary deficiencies, without necessarily being aware of it. Moreover, living in proximity to domesticated animals meant that their diseases became our diseases. Leprosy, plague, tuberculosis, typhus, diphtheria, measles, influenzas—all vaulted from goats and pigs and cows and the like straight into us. By one estimate, about 60 percent of all infectious diseases are zoonotic (that is, from animals). Farming led to the rise of commerce and literacy and the fruits of civilization but also gave us millennia of rotten teeth, stunted growth, and diminished health.
We forget how devastating many diseases were until quite recent times. Take diphtheria. Into the 1920s, before the introduction of a vaccine, it struck down more than 200,000 people a year in America, killing 15,000 of them. Children were especially susceptible. It usually started with a mild temperature and a sore throat, so at first was easily mistaken for a cold, but it soon became much more serious as dead cells accumulated in the throat, forming a leathery coating (the term “diphtheria” comes from the Greek for “leather”; the disease, incidentally, is correctly pronounced “diff-theria,” not “dip-theria”) that made breathing increasingly difficult, and the disease spread through the body, shutting down organs one by one. Death tended to follow swiftly. There were many cases of parents losing all their children in a single outbreak. Today diphtheria has become so rare—just five cases in the United States in the most recent decade measured—that many doctors would struggle to recognize it.
Typhoid fever was no less frightening and caused at least as much distress. The great French microbiologist Louis Pasteur understood pathogens better than anyone of his day but still lost three of his five children to typhoid fever.
Typhoid and typhus have similar names and symptoms but are different diseases. Both are bacterial in origin and marked by sharp abdominal pain, listlessness, and a tendency to grow confused. Typhus is caused by a rickettsia bacillus; typhoid is caused by a type of salmonella bacillus and is the more serious of the two. A small proportion of people infected with typhoid—between 2 and 5 percent—are infectious but have no symptoms of illness, making them highly effective, if nearly always unwitting, vectors. The most famous such carrier was a shadowy cook and housekeeper named Mary Mallon who became notorious in the early years of the twentieth century as Typhoid Mary.
Almost nothing is known of her beginnings. She was variously reported in her own day as being from Ireland, England, or the United States. All that can be said for certain is that from young adulthood Mary worked in a number of well-to-do households, mostly in the New York City area, and wherever she went, two things always happened: people came down with typhoid, and Mary abruptly disappeared. In 1907, after a particularly bad outbreak, she was tracked down and tested and in the process became the first person to be confirmed as an asymptomatic carrier—that is, was infectious but had no symptoms herself. So fearsome did this make her that she was held in protective custody, very much against her will, for three years.
She was released when she promised never again to take a job handling food. Mary, alas, was not the most trustworthy of souls. Almost immediately she began working in kitchens again, spreading typhoid to a number of new locations. She managed to elude capture until 1915, when twenty-five people developed typhoid at the Sloane Hospital for Women in Manhattan, where Mary had been working under an assumed name as a cook. Two of the victims died. Mary fled but was recaptured and spent the remaining twenty-three years of her life under house arrest on North Brother Island in the East River until her death in 1938. She was personally responsible for at least fifty-three cases of typhoid and three confirmed deaths, but possibly many more. The particular tragedy of it is that she could have spared her unfortunate victims if she had just washed her hands before handling food.
Typhoid may not worry people as it once did, but it still affects more than 20 million people a year around the world and kills between 200,000 and 600,000, depending on whose figures you rely on. The United States has an estimated 5,750 cases each year, about two-thirds brought in from abroad but nearly 2,000 acquired domestically.
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If you want to imagine what a disease might do if it became bad in every possible way, you could do no better than consider the case of smallpox. Smallpox is almost certainly the most devastating disease in the history of humankind. It infected nearly everyone who was exposed to it and killed about 30 percent of victims. The death toll in the twentieth century alone is thought to have been around 500 million. Smallpox’s astounding infectiousness was vividly demonstrated in Germany in 1970 after a youthful tourist developed it upon returning home from a trip to Pakistan. He was placed in hospital quarantine but opened his window one day to sneak a cigarette. This, it has been reported, was enough to infect seventeen others, some two floors away.
Smallpox only infects humans, and that proved to be its fatal weakness. Other infectious diseases—flus notably—can disappear from human populations but rest up, as it were, among birds or pigs or other animals. Smallpox had no such reservoir to retreat to as humans gradually persecuted it into smaller and smaller patches of the planet. At some point in the distant past, it had lost the ability to infect other animals in order to focus exclusively on humans. As it turned out, it chose the wrong enemy.
Now the only way any human can get smallpox is if we inflict it upon ourselves. Unfortunately, that has happened. In 1978, at the University of Birmingham in England, a medical photographer named Janet Parker went home from work early one afternoon in late summer complaining of a blinding headache. Soon she was very ill indeed—fevered, delirious, and covered in pustules. She had contracted smallpox via an air duct from a lab one floor below her office. There, a virologist named Henry Bedson had been studying one of the last smallpox samples on Earth still allowed for research. He was frantically working against a deadline before his own stocks were to be destroyed and evidently grew careless in keeping them safe. Poor Janet Parker died about two weeks after being exposed and in so doing became the last person on Earth to be killed by smallpox. She had actually been vaccinated against the disease twelve years earlier, but smallpox vaccine doesn’t last. When Bedson learned that smallpox had escaped from his lab and killed an innocent person, he went out to his garden shed and committed suicide, so in a sense he was smallpox’s last victim. The hospital ward on which Parker was treated was sealed off for five years.