Sandrine Belouzard, virologist and researcher, uses a microscope as she works in her epidemiology laboratory of the Infection and Imminence Center at the Pasteur Institute of Lille on Feb. 17, 2020 in Lille, France. The research institute has sequenced the genome of Coronavirus 2019-nCoV.
Sylvain Lefevre—Getty Images
The Faroe Islands are little volcanic spits of land poking out of the far north Atlantic Ocean. On the outer edges of Europe, they are isolated and cold; in 1846 they were one of the healthiest places on earth. But in that year, a carpenter, a native of the island, returned from Copenhagen with a bad cough. He had the measles. The virus had been absent from the Faroe Islands for more than 60 years, and, in the days before a measles vaccine, few of the island’s residents had immunity to the disease. Over the course of the next five months, 6,100 of the island’s 7,900 inhabitants fell ill. Over a hundred died.
Island populations are natural laboratories for epidemiology — the study of the population dynamics of infectious disease. And the measles epidemic of 1846 is of unusual interest because it marks the beginning of modern epidemiology. The Danish government, the distant sovereign over these tiny islands, dispatched Peter Panum, an observant young doctor with mutton-chop sideburns, to the scene. His report proved beyond a doubt that the disease was spread by direct contact. It is a seminal document of epidemiology.
Notably, the measles epidemic on the Faroes burned itself out in the space of five months. When the virus ran out of new hosts, the chain of infection was extinguished. Herein lies one of the most important lessons of the epidemic. The measles virus has a “critical community size” of 250,000, meaning that without a dense human population above that threshold, the virus will naturally drive itself to extinction, with nobody nearby left to get it and pass it on. The survival of the measles virus, as with any parasite, is intricately connected to the population dynamics of its host.
The history of a respiratory virus like measles can help to put the current coronavirus pandemic in perspective. Humans are distinctive animals in the number and nature of our pathogens — and especially in the number of respiratory viruses that infect us. COVID-19 is a new contender to join the permanent roster, but we are watching a pattern in human history repeat itself before our eyes.
What we often fail to recognize is that Homo sapiens serves as host to an unusual number of respiratory viruses. The sniffling, sneezing, coughing afflictions that stalk us from birth to old age are a distinct feature of life as a human being. In all, humans are afflicted by dozens of respiratory viruses that have evolved to specialize in the exploitation of us. That’s strange. Consider the disease pool of our closest primate relative, the chimpanzee. Because of our genetic relatedness, humans and chimpanzees have similar immune systems. Yet only about two dozen viruses have ever been identified in chimps, and many of these are in fact human viruses that have transiently infected chimpanzee populations (sometimes with horrific effect) when exposed to human observers. Chimps are the natural host of only a few viruses, and these are on the whole rather benign. Most of the parasites that are adapted to chimpanzees are worms or protozoa. The reasons for this pattern have to do with their population sizes and lifestyles. Chimps live in small groups and move frequently, which makes it impossible for specialized respiratory viruses that cause acute disease to adapt to them. In short, our chimpanzee cousins — who live in the jungle, eat raw monkey for breakfast, never bathe and make a habit of chewing on their own feces — suffer only a fraction of the viral diversity that we do.
