Q: It has been scientifically proven that a person having many sexual partners, engaging in unsafe sexual practices or re-using used syringes can contract AIDS. Also, if you agree with me, I would opine that the disease AIDS was never heard of until recently (we hadn't even heard of it about 20 years back). But didn't our ancestors (living more than 100-150 years ago) indulge in such activities? They definitely must have but why is that they never had the risk of getting infected with the HIV virus or AIDS? Why is it that AIDS has become prevalent in our society of late which wasn't the case, say, about 100-150 years ago?

A:New viruses and bacteria emerge from time to time as a result of mutations and genetic variations that occur as organisms evolve. Organisms also cross over from the animal kingdom and start to affect humans. It is likely that the HIV as we know it now has emerged relatively recently and so our ancestors may have been spared this problem.