The Australian virologist Sir Frank Macfarlane Burnet declared in 1962 that one of the most important social revolutions in history was the “virtual elimination” of infectious diseases. Burnet won the Nobel Prize for medicine; he was a brilliant man. But he was wrong.
Covid-19 is probably the third coronavirus to have jumped from bats to humans since 2002. Severe Acute Respiratory Syndrome killed at least 774 people after emerging in China’s Guangdong province, most likely originating in horseshoe bats before spreading to humans via civet cats. In 2012, Middle East Respiratory Syndrome, first identified in Saudi Arabia, spread to humans from bats via camels.
Between 1940 and 2004, no fewer than 335 new infectious diseases emerged in humans, with 60 per cent leaping from animals in so-called spillover events, according to an analysis published in Nature. Of the spillovers, 72 per cent came from wildlife.
Back in 1962, it was easy to believe Burnet was right. In the 19th century, better nutrition, rat-proof housing, clean water and sewerage systems had dramatically curtailed infectious diseases. By the time of Burnet’s prognosis, antibiotics and vaccines had produced further miracles. Within two decades, smallpox would be eliminated and polio driven from most advanced nations.
Burnet had not fully considered what we might call the paradox of progress. For every human advance, from speed of travel to intensity of farming, there are hidden dangers.
Precisely the same factors that allow us to create food surpluses and mRNA vaccines open us up to the risk of pandemics worse than the one we are living through now. The more humans tip the world into disequilibrium, through deforestation, the destruction of biodiversity and the raising of atmospheric temperatures, the more threat to us pathogens will pose.
Consider agriculture. Human civilisation as we know it would not have been possible if hunter gatherers had not settled in villages. But those conditions were also ideal for pathogens to jump from domesticated animals into humans. Influenza may have evolved from avian flu while measles came from the rinderpest virus in cattle. Now many of us live in dense megacities perfect for the exchange of knowledge, but also perfect for pathogenic spread.
Better farming techniques allowed humans to raise crop yields and feed an increasingly rampant species. In defiance of Thomas Malthus, our numbers have ballooned from 900m when he was positing the limits of the human population at the turn of the 19th century to 7.8bn today. Now, only a few mammals, including cattle and mice, outnumber us. That blistering replicatory success has a downside. From the parasite’s point of view, what better strategy than infecting humans? In the words of Paul and Anne Ehrlich, who warned of the “population bomb” more than 50 years ago, infiltrating homo sapiens amounts to “winning a jackpot”.
Human encroachment is another risk factor. As settlements move closer to forests and the wildlife within, pathogens are more likely to cross species. Marburg’s disease and HIV, a virus that has killed some 33m people, came from primates. Zoonotic diseases are not restricted to tropical countries. The fanning out of America’s urban populations into suburbs created the conditions in which Lyme disease, a potentially debilitating illness, can spread from ticks to humans. Climate change is altering the range in which potential vectors of disease can survive.
Speed is also beloved by pathogens. The third bubonic plague pandemic may have been confined to southern China had it not reached the ports of Canton and Hong Kong in 1894. The advent of steamships transported the disease to the world’s great ports, killing more than 10m people. Aeroplanes are pathogen accelerators. Today, a new disease can jump from animal to human in Latin America, central Africa or south-east Asia and be anywhere on earth by tea time.
Much of what humans need to do is obvious. We must stop trading exotic species, especially if they end up on the dinner table. We must strictly regulate markets where animals are sold. We should stop farming mink and other animals for fur.
We must invest in early warning systems so that outbreaks of diseases anywhere can be quickly detected and controlled. We must pour money into vaccine platforms and vaccine production capacity so that, if diseases do break through, they can rapidly be neutralised. Above all, we must halt the destruction of nature that is unleashing pathogenic forces we cannot control.