Additionally, in order to maximise transmission, viruses often naturally undergo attenuation (i.e. virulence is reduced) so that infected animals can survive long enough to infect other animals more efficiently. However, as attenuation takes time to achieve, new host populations will not initially benefit from this phenomenon. Moreover, as zoonotic viruses also naturally exist in animal reservoirs, their survival is not dependent on transmission between new hosts; this means that emergent viruses are even more unlikely to attenuate for the purpose of maximal transmission, and they remain virulent.