AIDS is caused by [D] Virus. Specifically, it is caused by the Human Immunodeficiency Virus (HIV), which attacks the immune system, leading to Acquired Immunodeficiency Syndrome (AIDS). HIV primarily targets CD4 cells, weakening the immune system and increasing susceptibility to infections and certain cancers. Unlike bacteria [A], fungi [B], or worms [C], which can cause various diseases, AIDS specifically results from HIV infection, transmitted through blood, semen, vaginal fluids, and breast milk. Effective antiretroviral therapy (ART) can manage HIV, but there is currently no cure, making prevention crucial in controlling the spread of the virus.