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A recent issue of Science contains an article describing the worldwide decline of amphibian populations due to the international spread of chytridiomycosis, a lethal fungal disease, largely as a result of the trade in amphibians as pets. The greatest declines are reported amongst formerly isolated populations in the wet forests of the Americas and Australia. Few of the infected populations are expected to recover.

The global movement of people and goods brings exotic diseases to most ecosystems on Earth, including Ebola, Zika, and COVID-19 to humans in North America. For the human species, the problem is exacerbated by the concentration of our numbers in dense urban populations. As a social species, we frequent large gatherings of people, such as meetings and sporting events. We travel in sealed aluminum tubes, in which we function as a laboratory culture for our pathogens.

The most vulnerable populations of animals and humans are those who have not previously encountered the disease agents and thus have no evolutionary resistance to it. The phenomenon is known as the “virgin soil effect”, which explains the vulnerability and devastation of native Americans following the arrival of European diseases. (It is likely that syphilis traveled in the opposite direction with similar impacts in Europe). COVID-19 appears to have arisen in wildlife in China, for which the rest of the world had no experience that might have led to the evolution of antibodies. Through globalization, we have homogenized the world experience, whereas in nature, a higher diversity of organisms persists when habitats and populations are isolated.

Our vulnerability to exotic disease is often exacerbated by the widespread use of antibiotics and antifungal compounds in agriculture. The effect is similar to the appearance of DDT-resistant mosquitoes in areas where DDT was broadcast to prevent malaria. As a virus COVID-19 is unaffected by such measures, but its spread amongst the human population and our vulnerability to an exotic pathogen are parallel.

Not since the plague, also known as the Black Death, arrived in Europe in the early 1300s has the human population been more vulnerable to exotic disease. Black Death is estimated to have killed 30 to 60% of the population of Europe. With international transport of people and goods, dense human populations in cities, and a social structure based on close interactions among us, the human population is more vulnerable to catastrophic pandemics than at any time since the Black Death.

Fear the fever.