Its not smallpox. It is related to a group of simian viruses, as in comes from monkeys. The degree of mutation suggests strongly that it first cropped up in the 50's. How it was transferred from ape to human is a subject of hot debate. The theory you are referring to is that doctors in africa producing cheap vaccines locally (I think it was polio not smallpox) used apes and ape materials to gorw the cultures in and introduced HIV by accident. They strongly deny this. Mind you a single bite from a chimp to a horny bushmeat hunter would have started things off, so tracking the original source will be nightmarish.