Spillover infection

Spillover infection, also known as pathogen spillover and spillover event, occurs when a reservoir population with a high pathogen prevalence comes into contact with a novel host population. The pathogen is transmitted from the reservoir population and may or may not be transmitted within the host population. Due to climate change and land use expansion, the risk of viral spillover is predicted to significantly increase. Spillover is a common event; in fact, more than two-thirds of human viruses are zoonotic. Most spillover events result in self-limited cases with no further human to human transmission, as occurs, for example, with rabies, anthrax, histoplasmosis or hidatidosis. Other zoonotic pathogens are able to be transmitted by humans to produce secondary cases and even to establish limited chains of transmission. Some examples are the Ebola and Marburg filoviruses, the MERS and SARS coronaviruses and some avian flu viruses. Finally, some spillover events can result in the final adaptation of the microbe to humans, who can become a new stable reservoir, as occurred with the HIV virus resulting in the AIDS epidemic and with SARS-CoV-2 resulting in the COVID-19 pandemic. If the history of mutual adaptation is long enough, permanent host-microbe associations can be established resulting in co-evolution, and even permanent integration of the microbe genome with the human genome, as it is the case of endogenous viruses. The closer the two target host species are in phylogenetic terms, the easier it is for microbes to overcome the biological barrier to produce successful spillovers. For this reason, other mammals are the main source of zoonotic agents for humans. For example, in the case of the Ebola virus, fruit bats, are the hypothesized zoonotic agent. During the late 20th century zoonotic spillover increased as the environmental impact of agriculture promoted increased land use and deforestation, changing wildlife habitat.