Since May, 2022, clusters of monkeypox infections have caused global concern. At present, this concern has been tempered by the fact that, even when uncontrolled, the number of infections is growing slowly, indicating a reproductive number (R) not much larger than unity. However, the effect of R on the probability of evolution might not be obvious. We suggest that, compared with zoonotic pathogens with large R values, those pathogens with R values just above 1, such as monkeypox virus, have a higher probability of evolution during the timeframe in which the number of cases remains low. Waiting until the number of cases is high would give monkeypox virus—or any emerging pathogen—the opportunity to adapt substantially to humans.

Population growth, ecological degradation, and climate change have increased the frequency of contact between humans and other animals, wild and domestic alike. The consequences include greater opportunities for pathogens to cross species barriers. Recent high-profile cases include Ebola virus (from bats), MERS coronavirus (bats or camels), SARS-CoV-1 and SARS-CoV-2 (bats), and monkeypox (rodents). After a zoonosis spills over into humans, subsequent evolution of the virus results in higher transmission, making control more difficult, and causing unpredictable changes in disease severity, as seen with different variants in the ongoing SARS-CoV-2 pandemic.

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