The answer would be D. HIV.
HIV stands for human immunodeficiency virus which is transmitted through sexual contact. It can develop into acquired immunodeficiency syndrome (AIDS) which is oftentimes fatal.
HIV interferes with the body's ability to fight infections which spells trouble for anybody dealing with the virus. Not being able to fight infection means that anything that could make someone sick could become fatal. A simple cold could be the thing that kills someone because their body is literally incapable of fighting the infection.