> The only explanation that makes sense to me is that HIV has emerged previously, but only in dead-end strains that have very low infectivity, rather like the H

> The only explanation that makes sense to me is that HIV has emerged previously, but only in dead-end strains that have very low infectivity, rather like the HIV-1 groups N and P mentioned in the article. Or that modern innovations like transportation and so on made it possible for it to spread before it died out.