According to Kyle Harper in "Plagues Upon the Earth," Why are respiratory infections in other apes practically, naturally non-existent? ( whereas they are quite common among rodents and bats)

a. Other apes don't live long enough to be exposed
b. They have more robust respiratory immunity, compared to humans
c. A combination of physiology and population density
d. They are persistently infected with worms, which are protective against respiratory ailments