Figure 1. Many viruses naturally circulate within animals (blue); some of these can spill over into humans. These wildlife viruses that infect humans are called zoonotic viruses. The transmission cycle ends for those viruses that humans cannot pass on. However, those viruses that evolve a way to transmit among humans (purple) can cause a human epidemic, and, if the disease spreads across continents, a pandemic.
Credit: Samantha Wisely, UF/IFAS