A spillover occurs when a pathogen infects a new host species (2,3). The vast majority of spillovers will not lead to an outbreak or pandemic. However, for pathogens with pandemic potential, each spillover into a human is an opportunity to launch a pandemic.
A spillover is when a virus or pathogen that usually lives in animals crosses over and infects humans. It happens when people come into close contact with animals that carry the virus or with things contaminated by them. Most spillovers don’t cause large outbreaks, but each one gives the virus a chance to adapt and spread.