HIV attacks the immune system by targeting immune cells called CD4 cells or T cells. This weakens the immune system and makes it harder for the body to fight infections and diseases. HIV is most commonly transmitted through unprotected sex, contaminated needles, or from mother to child during pregnancy, childbirth or breastfeeding. While HIV affects people of all ages, those most at risk are men who have sex with men and injection drug users. Free or low-cost testing and treatment options are available at certain clinics in Los Angeles.