Table 1. Virus traits potentially relevant for capacity to emerge and cause disease in human populations*.

Trait	Definition
Reservoir host relatedness	Viruses derived from specific host taxa (e.g., other primate species might be of increased concern)
Virus relatedness	Particular virus taxa might be predisposed to infect, cause disease, and transmit among humans
Virus host range	Viruses with a broad or narrow host range might be of greatest concern
Evolvability	Higher substitution rates might make it easier for some viruses to adapt to human hosts
Host restriction factors	Host factors, many still to be identified, are a barrier to viral infection and help determine which viruses can and cannot emerge
Transmission route	Certain transmission routes might predispose viruses to emerge in humans
Virulence	Certain virus or host factors might determine whether a virus causes mild or severe disease in humans
Host−virus coevolution	Lack of a shared evolutionary history might be associated with higher virulence

*Adapted from Morse et al. (3).