Human immunodeficiency virus (HIV) is the cause of a chronic progressive disease which, among the patients, clinically varies from an asympthomatic disease recognizable by laboratory test results (HIV-infection) to a state of completely distroyed immune system called acquired immune deficiency syndrome (AIDS). There are three common ways of transmission of HIV infection: sexual transmission, blood transmission and transfer from infected mother to child during the pregnancy, delivery or breast-feeding. As the infection is manifested with a wide range of clinical syndromes, it is usually undetected for a long time. The diagnosis of the infection is made by immunoenzyme ELISA test and then confirmed by Western blot test. HIV-infection can be treated by the combination of 2 nucleoside/ nucleotide reverse transcriptase inhibitors with one of the following drugs: non nucleoside reverse transcriptase inhibitor, integrase inhibitor or protease inhibitor combined with low-dose ritonavir. HIV/AIDS disease has a pandemic proportions. The largest number of infected people lives in the Sub-Saharan Africa (24.7 million). The overall period of monitoring in Croatia (1985-2014) shows that there have been 1208 people infected with HIV and 441 of them have had AIDS. 212 HIV infected people died and 180 of them were at the stage of AIDS. The average number of new cases of HIV in Croatia in the last 5 years has been around 80 people per year ( range from 70 to 92). The total number of cases of infection shows that there have been 1042 infected men (86%), and 166 infected women (14%). The most of the new cases of HIV/AIDS infection (53%) have been diagnosed when patients were 25 to 39 years old. The most common way of HIV transmission in Croatia is through homosexual intercourse (59,1%), followed by heterosexual intercourse (29,2%), by intravenous drug use (5,9%) and by transfusion of the infected blood (1,4%). Vertical transmission of HIV has been registrated in 1.2%, and unknown path of transmission in 3.3% of cases. The prevention of HIV infection is necessary to reduce the risk of spreading of the disease in the future and to maintain the low level of the new HIV cases.