The causative agent of AIDS is the Human immunodeficiency virus (HIV), which specifically targets and destroys CD4+ T lymphocytes, leading to progressive immune system failure. This immunodeficiency makes patients vulnerable to opportunistic infections and certain cancers that define AIDS. Unlike the other viruses listed, HIV directly impairs immune function by integrating into host DNA and causing chronic infection. Understanding HIV’s role is crucial for diagnosis, treatment, and prevention of AIDS.

Reference: Harrison's Principles of Internal Medicine, J. Larry Jameson, 20th Edition.