The Animal Origins and Evolutionary Strategies of Human Epidemic Diseases

Key Insight

Many of humanity's deadliest diseases, including smallpox, flu, tuberculosis, malaria, plague, measles, and cholera, originated from diseases of animals. A specific incident involving a patient who contracted pneumonia from repeated intercourse with sheep illustrates the direct transfer of pathogens from animals to humans. Similarly, the explosively spreading human disease AIDS appears to have evolved from a virus residing in wild African monkeys, underscoring the broad significance of animal-human disease transfer.

From a microbe's evolutionary perspective, disease 'symptoms' are often clever strategies to ensure transmission and spread. Genital sores from venereal diseases like syphilis facilitate direct transfer between hosts, while influenza and pertussis microbes induce coughing or sneezing to launch themselves toward new victims. The cholera bacterium causes massive diarrhea to broadcast itself into water supplies, and the rabies virus drives infected dogs into a frenzy of biting, actively enlisting the host to infect others.

While it seems counterintuitive for a microbe to kill its host, this is typically an unintended by-product of optimizing transmission efficiency. For instance, an untreated cholera patient may die from severe fluid loss, but the bacterium benefits from being massively broadcast while the host is still alive. Evolution favors microbes that, on average, infect more than one new victim, even if the initial host succumbs. This is evident in the evolution of syphilis and myxomatosis, where less lethal strains that keep hosts alive longer are more successful at spreading.