Human immunodeficiency virus (HIV) is a virus that attacks an infected person’s immune system. HIV destroys CD4 cells, a type of white blood cell that fights infection. If left untreated, a person infected with HIV is likely to develop other infections and/or infection-related cancers. HIV can develop into acquired immunodeficiency syndrome (AIDS), which is the most advanced and life-threatening stage of HIV. A person is considered to have AIDS when their CD4 count is below 200 cells per cubic millimeter of blood (cells/mm3)—healthy people’s counts run from 500–1,600 cells/mm3—or when they develop one or more opportunistic infections. Opportunistic infections are infections caused by an organism that does not normally cause disease in healthy people.

HIV can be transmitted from an infected person through blood, semen, vaginal and other body fluids, and breast milk. HIV can be transmitted through unprotected sex; infected needles, syringes and other infection equipment; and from an infected mother to her child during pregnancy, childbirth or nursing.

People can protect themselves from contracting HIV by:

  • Getting tested and knowing their partner’s HIV status
  • Using a condom every time they have sex
  • Using a clean needle and syringe, and never sharing equipment when injecting drugs
  • Taking pre-exposure prophylaxis (PrEP)—a daily pill demonstrated to be more than 90% effective in preventing HIV for those who are at risk and do not have HIV

Pregnant women and new mothers who are HIV-positive can reduce the risk of transmitting the infection to their babies through HIV treatment.

For HIV-negative men, voluntary medical male circumcision (VMMC)—a safe, minor procedure—can reduce their risk of female-to-male HIV transmission by 70%.

Health care workers should follow standard precautions to protect themselves.

HIV-positive people can reduce their risk of transmission by viral suppression through antiviral treatment.

While there is no cure for HIV, antiviral treatments can reduce the amount of virus in a person’s system, which helps the immune system to recover. This also helps reduce the risk of transmitting the virus to others. The World Health Organization recommends that once a person is diagnosed with HIV, they should start treatment.

The US Centers for Disease Control and Prevention states that a person with HIV who takes HIV medicine as prescribed and gets and stays virally suppressed (the virus is undetectable) can stay healthy and has effectively no risk of sexually transmitting HIV to HIV-negative partners.

HIV testing is the only way a person can know if they have HIV. For people living with HIV, testing services can be a critical entry point for lifesaving care and treatment. Once a person is diagnosed and seeks care, they have a better chance of living a long, healthy life. Testing services can also link people who test negative to other prevention services, such as PrEP, VMMC, condoms and risk reduction counseling. When a person knows their status, they can keep themselves and their sexual partners healthy.

According to the most recent Joint United Nations Programme on HIV/AIDS data, key populations (sex workers and their clients, men who have sex with men, people who inject drugs, transgender people) accounted for 55% of new HIV infections globally.

Around half of all new infections occurred in sub-Saharan Africa, where women and girls account for more than 77% of all new HIV infections among young people and key populations account for 25% of new infections.