HIV (human immunodeficiency virus) is a virus that attacks the immune system, specifically targeting CD4 cells, which are essential for fighting infections. Over time, untreated HIV can weaken the immune system to the point that it progresses to AIDS (acquired immunodeficiency syndrome), the most advanced stage of the infection. AIDS is characterized by severe immune damage and the development of opportunistic infections or certain cancers that the body would normally be able to resist. HIV is transmitted through contact with infected blood, semen, vaginal fluids, rectal fluids, or breast milk. Early infection may cause flu-like symptoms, but many individuals may have no symptoms for years while the virus continues to damage the immune system. Early diagnosis and consistent medical care are crucial for preserving immune function and preventing progression to AIDS.