Retroviruses procreate anytime.
Second, whereas most organisms have only one reproductive choice, HIVs have two choices. In retroviruses like HIV that reproduce quickly and often, recombination allows major change in a short time. It can eliminate mutations from the father and mother that are deleterious to the virus. New subtypes result when two nonidentical parent subtypes infect a single host cell. Once cross-species transmission had occurred, a human epidemic was probably inevitable.