This is an important question. The best place to start with respect to this question is by examining what HIV stands for. HIV stands for human immunodeficiency virus (HIV). As the name suggests, HIV weakens our immune systems. In other words, its weakens the human body defense system, so that it is not able to fight off diseases and viruses. So, a person with HIV can die of pneumonia, which a typical person can fight off.
More specifically, HIV destroys a type of defense cell in the body called a CD4 helper lymphocyte. For this reason the body is compromised. In short, these people become immunodeficient, to use a medical term. The outcome of this is that people begin to get sicknesses, which can ultimately lead to their death. This compromise is what makes HIV so deadly.