The coronavirus is just the latest of many deadly diseases to cross over to human beings from other species.
February 6, 2020
Earlier this week, the still-rising death toll in mainland China from the coronavirus surpassed the 349 fatalities recorded during the 2003 SARS epidemic. Although both viruses are believed to have originated in bats, they don’t behave in the same way. SARS spread slowly, but its mortality rate was 9.6%, compared with about 2% for the swift-moving coronavirus.
Statistics tell only one part of the story, however. Advances in the genetic sequencing of diseases have revealed that a vast hinterland of growth and adaptation precedes the appearance of a new disease. Cancer, for example, predates human beings themselves: Last year scientists announced that they had discovered traces of bone cancer in the fossil of a 240-million-year-old shell-less turtle from the Triassic period. This easily surpasses the oldest example of human cancer, which was found in a 1.7 million-year-old toe bone in South Africa. The findings confirm that even though cancer has all kinds of modern triggers such as radiation poisoning, asbestos and smoking, the disease is deeply rooted in our evolutionary past.
Unlike cancer, the majority of human diseases are zoonotic, meaning that they are passed between animals and people by viruses, fungi, parasites or bacteria. The rise of agriculture around 10,000 years ago, which forced humans into close contact with animals, was probably the single greatest factor behind the spread of infectious disease.
Rabies was one of the earliest diseases to be recognized as having an animal origin. The law code of Eshnunna, a Mesopotamian city that flourished around 2000 B.C., mandated harsh punishments against owners of mad dogs that bit people. Lyme disease was only identified by scientists in 1975, but it too was an ancient scourge. Ötzi, the 5,300-year-old mummy who was discovered in the Tyrolean Alps with cracked ribs and an arrow wound in his shoulder, was an unlucky fellow even before he was killed. DNA sequencing in 2010 revealed that while he was alive, Ötzi was lactose intolerant, had clogged arteries and suffered from Lyme disease.
Smallpox, which was eradicated by the World Health Organization in 1979, had been one of the most feared diseases for most of human history, with a mortality rate of 30%. Those who survived were often left with severe scarring; the telltale lesions of smallpox have been identified on the mummy of Pharaoh Ramesses V, who died in 1145 B.C. The disease is caused by the variola virus, which is thought to have crossed over to human beings from an animal, likely a rodent, in prehistoric times.
Although it can’t be proved for certain, it is likely that smallpox was behind the terrible plague that killed 20% of the Athenian population in 430 B.C. The historian Thucydides, who lived through it, described the agony of those infected with red pustules, the dead bodies piled high in the temples and the scars left on the survivors. He also noticed that those who did survive acquired immunity to the disease.
Thucydides’s observation turned out to be the key to one of humanity’s greatest weapons against infectious disease, vaccination. But apart from smallpox, the only eradication programs to have made some progress have been against viruses transmitted from human to human, such as polio and measles. Meanwhile, since the 1970s more than 40 new infectious diseases have emerged from the animal realm, including HIV, swine flu and Zika. And those are just the ones we know about.