Human immunodeficiency virus (HIV) is a virus that attacks the body's immune system, specifically targeting CD4 cells, which are crucial for fighting infections. If left untreated, HIV can lead to acquired immunodeficiency syndrome (AIDS), a condition where the immune system becomes severely weakened, making the body more vulnerable to infections and diseases. HIV is primarily transmitted through 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 (ART) can help manage the virus and improve the quality of life for those infected.