Which virus is known for gradually destroying the body's immune system?

The Human Immunodeficiency Virus (HIV) is recognized for its role in gradually attacking the immune system, specifically targeting CD4 cells, which are vital for immune response. Over time, as HIV destroys these cells, the body's ability to fight off infections and diseases is significantly impaired. This progression can eventually lead to Acquired Immunodeficiency Syndrome (AIDS), where the immune system is severely compromised.

In contrast, the other viruses mentioned have different impacts on the body. The influenza virus primarily causes respiratory illness and does not have a long-term debilitating effect on the immune system similar to HIV. Hepatitis B virus primarily affects the liver rather than the immune system directly. Cytomegalovirus, while it can cause illness, does not specifically target and destroy immune cells in the way that HIV does. Therefore, HIV is distinctly characterized by its gradual destruction of the immune system.