That's not strictly "more efficient", at least in that notable zoonotic (https://en.wikipedia.org/wiki/Zoonosis https://en.wikipedia.org/wiki/Zoonosis) viruses

That's not strictly "more efficient", at least in that notable zoonotic (https://en.wikipedia.org/wiki/Zoonosis https://en.wikipedia.org/wiki/Zoonosis) viruses tend to be noted because they're more lethal when they jump from an animal to humans. They've been observed to adapt over time to be less, or less quickly lethal, because those sorts of adaptational mutations allow more viral spread. Of course we don't always notice this because if e.g. an influenza were to adapt to the level of causing a "cold", we wouldn't necessarily notice. See the other comment on how SIV just might have "mutated overnight", in that modern conditions allowed it to spread: https://en.wikipedia.org/wiki/HIV#Origins https://en.wikipedia.org/wiki/HIV#Origins