Fig. 1 A possible transmission cycle of a novel virus with pandemic potential
Novel viral diseases are potentially serious threats to public health, because tests, vaccines and effective treatments are not readily available, as typified by the ongoing COVID-19 pandemic. Many highly pathogenic viruses, including Ebola and influenza, as well as SARS-CoV-2 originated in other animal host species, and were transmitted to humans from the original host, possibly via an intermediate animal host (Fig. 1). Most viruses that transfer to new host species cause limited outbreaks, but when a virus both acquires the ability to transmit efficiently from human to human and throughout communities and countries, and also retains a high level of pathogenicity, then the toll on human populations can be significant.