There’s the fact that most viral spillovers into humans don’t actually transmit well, and the fact that if they do but burn out it doesn’t really matter on the large scale. The latter has definitely changed.
HIV is an interesting case study. Genetically, we can tell it leaped into humans from great apes TWICE in the 1920s (HIV-1 and HIV-2) in a way that kept transmitting around the globe. This probably had to do with the urbanization of sub-Saharan Africa—the sudden sucking of huge numbers of rural people with contact with bushmeat into globally connected urban centers. Presumably it had been transmitting into people forever, just always burning out.
A few years back there was an antibody study in a village near a bat cave in China that found that 0.5% of the people that lived a few hundred meters from the cave showed antibodies against SARS-like viruses.
There’s the fact that most viral spillovers into humans don’t actually transmit well, and the fact that if they do but burn out it doesn’t really matter on the large scale. The latter has definitely changed.
HIV is an interesting case study. Genetically, we can tell it leaped into humans from great apes TWICE in the 1920s (HIV-1 and HIV-2) in a way that kept transmitting around the globe. This probably had to do with the urbanization of sub-Saharan Africa—the sudden sucking of huge numbers of rural people with contact with bushmeat into globally connected urban centers. Presumably it had been transmitting into people forever, just always burning out.
A few years back there was an antibody study in a village near a bat cave in China that found that 0.5% of the people that lived a few hundred meters from the cave showed antibodies against SARS-like viruses.