Barriers to spillover. Exposure of a host to a potential pathogen is insufficient to lead to an infectious cycle. There are multiple barriers that the pathogen needs to overcome before a successful infection is established. Some, such as the ecology and natural history of host and pathogen, play out in the outside world (the external environment). But even when a pathogen successfully enters into the internal environment of a potential host, there are still multiple barriers it needs to overcome, such as the host's ability to neutralise the pathogen through its immune response or lack of molecular compatibility, before a successful infection cycle is in place. Source: Modified from Plowright and colleagues (2017).