Fig. 1: The potential zoonotic transmission cycle for coronaviruses.

Coronaviruses may transmit naturally (black arrows) among humans, bats and other wildlife (such as racoon dogs, hedgehogs, pangolins, palm civets, camels (as is known for MERS-CoV) and mink)¹⁵⁸. Human interventions may amplify the spread (red arrow). Transmission cycles may be amplified in urban areas that are normally at a minimal risk of exposure, increasing transmission to humans and accelerating an outbreak scenario. (1) Natural zoonotic infection cycles from domestic animals or wildlife (including bats) to humans and vice versa; human populations at risk include bat guano farmers, or individuals living and working in areas that overlap with bat habitats. (2) Natural enzootic cycle between different species of wildlife (including bats), and domestic animals and wildlife. (3) Amplification and spread between overlapping bat populations—as, for example, seen among species in the Rhinolophidae and Hipposideridae for SARS-related coronaviruses¹⁵⁹. (4) Amplified zoonotic infections and spread to urban areas via human interventions, including wildlife trade and increased urbanization. (5) Anthropozoonotic infections from humans back to domestic animals or wildlife (for example, as in mink farming⁵⁰). (6) Human migration patterns facilitate spread to urban areas (for example, during holiday seasons¹⁶⁰). (7) Amplified viral spread among humans or animals and humans in dense urban settings.