HIV: a virus that, if left untreated, can weaken a person’s immune system so that the person cannot fight off many everyday infections. Human immunodeficiency virus (HIV) can be transmitted through exposure to the blood, semen, vaginal fluid or breast milk of a person living with HIV. When a person contracts HIV, HIV medicine (called antiretroviral therapy or ART) can be taken to reduce the amount of the virus (viral load) in the person’s body such that the virus cannot be detected in a blood test. This is called an undetectable viral load. When undetectable status is achieved and maintained, the person living with HIV cannot transmit HIV. HIV, if left untreated, may lead to AIDS.