But the 1918 Spanish flu has, as far as I know, legitimately died out. Lots of people like saying that in a sense it’s still with us. This NEJM paper (with a celebrity author!) points out that it’s the ancestor of all existing flu strains. But most of these flu strains are less infectious than it was. This didn’t make sense to me the first, second, or third time I asked about it: why would a flu evolve into an inferior flu? Sure, it might evolve into a less deadly flu because it’s perfectly happy being more infectious but less deadly. But I think the Spanish flu was also especially infectious; so why would it evolve away from that?
One possible answer is “because by 1919, everyone had immunity to the 1918 flu, so it evolved away from it – and now nobody has immunity, but it lost the original blueprint.” The 1918 flu was a really optimal point in fluspace, but during all of history up until 1918, the flu’s evolutionary hill-climbing algorithm didn’t manage to find that point, and since flu has no memory it’s not going to be any easier for it to find it the second time, after it evolved away from it. So plausibly, existing flus are strictly worse at their job than Spanish flu was, and digging up an intact copy of the latter would be really bad.
And then there’s smallpox. No mystery why smallpox died out – we killed it. But then we stopped vaccinating people against it, and now if it comes back it would be really bad.
In the new world, inoculation had a very rough reception. When John Dalgleish and Archibald Campbell began inoculating individuals in Norfolk, Virginia, an angry mob burned down Campbell’s house. Similar incidents occurred in Salem and Marblehead, Mass. In Charleston, S.C., an inoculation control law of 1738 imposed a fine of £500 on anyone providing or receiving inoculation within two miles of the city. A similar law was passed in New York City in 1747.
The measures in New England were so draconian that Benjamin Waterhouse noted the paradox: “New England, the most democratical region on the face of the earth voluntarily submitted to more restrictions and abridgements of liberty, to secure themselves against that terrific scourge, than any absolute monarch could have enforced.” (This, strangely prescient, anticipates the current debate about liberty versus public health). It was in the middle colonies — Maryland, Pennsylvania, New Jersey — that inoculation was most tolerated in the second half of the 18th century. That’s why Jefferson made the long journey to Philadelphia to be inoculated in 1766.
Jefferson first became aware of the discovery of a true smallpox vaccine from the newspapers he read in Philadelphia and the new capitol in Washington, D.C. Then, on Dec. 1, 1800, just after Jefferson’s election to the presidency, Benjamin Waterhouse sent him his pamphlet on the vaccine with a lovely cover letter saying that he regarded Jefferson as “one of our most distinguished patriots and philosophers.” Jefferson responded immediately, thanking Waterhouse for the publication and declaring, with his usual grace, that “every friend of humanity must look with pleasure on this discovery, by which one evil the [more] is withdrawn from the condition of man: and contemplating the possibility that future improvements & discoveries, may still more & more lessen the catalogue of evils. in this line of proceeding you deserve well of your [country?] and I pray you to accept my portion of the tribute due you.”
Which microbe was responsible for the Antonine Plague remains unclear, though most specialists believe that the likeliest culprit is an ancestor of the smallpox virus. The Antonine Plague is one example of a broader lesson that becomes clear in the study of human disease: Many of the most vicious microbes of human history are not altogether very old. They emerged and evolved on human time scales, in recent millenniums and centuries — and in response to the opportunities we inadvertently presented them. A second lesson is that human health and animal health are inseparable. Our relationship with the environment reverberates back upon us, sometimes with destructive force.
The smallpox virus is less than 2,000 years old. The Antonine Plague may well represent an early stage of its evolution as a human pathogen. Like many viruses, the agent of smallpox belongs to a family many of whose representatives infect small mammals, like rodents. As human societies expand, and become more interconnected, we collide with animals and their diseases. Evolution relentlessly experiments with adaptations to new hosts, and some of these experiments unfortunately prove successful.
The Antonine Plague was such an experiment. Even without understanding the microbiology of the disease, the Romans knew that the Antonine Plague had come from without, that it was something new that had appeared with terrific fury. They believed that the pestilence had been unloosed by their own soldiers on campaign beyond Roman borders, inside what is now Iraq. More likely, the germ simply spread along the bustling trade routes that connected virtually the entire Old World. The Romans carried on a vigorous commerce with East Africa, the Near East and India and China beyond. As it happens, the first documented direct contact between Rome and China fell in the very year the Antonine Plague broke out under Marcus Aurelius. Though nothing compared with our “flat” world, the Romans lived through one of the most important phases in the long history of globalization. Then as now, exposure to disease was one of its unintended consequences.