The rare epidemics of viral diseases originating in animals would have been short-lived because the viruses were not fully adapted to humans and the human populations were too small to maintain the chains of infection.
Loomade viiruste harva esinenud epideemiad olid lühiajalised, kuna viirused ei olnud inimestega täielikult kohanenud ja inimeste populatsioonid olid liiga väiksed, et nakkusliini säilitada.