Researchers have discovered a genetic likely related to increased resistance to smallpox and plague in europe about 5,000 years ago that reduces susceptibility to

The answer is: mutation; HIV infection

Virus could not live on the host that does not provide suitable environment for it to live.

From the discovery , the researches try to propose an argument that in the past our body is not suitable environment for HIV virus to live. When our body experience a mutation to become resistance to smallpox or plague, our body change into an environment that is suitable for the HIV infection.  This become the reason why HIV become more prevalent since 500 years ago.