HIV infection is caused by the human immunodeficiency virus, which attacks the body's immune system, specifically the CD4 cells (T cells). If left untreated, HIV can lead to AIDS (acquired immunodeficiency syndrome), a condition where the immune system becomes severely weakened, making it difficult for the body to fight off infections and diseases. HIV is primarily transmitted through contact with infected bodily fluids, such as blood, semen, vaginal fluids, and breast milk. Common routes of transmission include unprotected sexual contact, sharing needles, and from mother to child during childbirth or breastfeeding. Early diagnosis and treatment with antiretroviral therapy can help manage the infection and improve quality of life.