Infectious diseases prevalent in humans and animals are caused by pathogens that once
emerged from other animal hosts. In addition to these established infections, new
infectious diseases periodically emerge. In extreme cases they may cause pandemics
such as COVID-19; in other cases, dead end infections or smaller epidemics result.
Established diseases may also re-emerge, for example by extending geographically or
by becoming more transmissible or more pathogenic. Disease emergence reflects dynamic
balances and imbalances, within complex globally-distributed ecosystems comprised
of humans, animals, pathogens, and the environment.Source: www.cell.com