Question

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 ________.

Answer 1

Researchers have discovered a genetic MUTATION likely related to increased resistance ................................ that reduces susceptibility to HIV INFECTION.
In 2005, Biologists at the University of Liverpool had discovered how the plagues of the middle age had made about 10% of European resistant to HIV infection. These individuals have genetic mutations which prevent the HIV virus from entering the cells of their immune system. 

Answer 2

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.