HIV stands for Human Immunodeficiency Virus that causes gradual decrease of humane immune response. Persons infected by HIV virus are „HIV positive“. In particular, the virus attacks a certain group of white blood cells, T-lymphocytes, which have a central role in the immune system of the human body. The virus multiplies in these cells and the viral nucleic acid (DNA) becomes part of the cell DNA (integrates into the cell DNA); the virus damages the function of these cells and subsequently kills them. Infection by HIV means a lifelong infection.

With the decreasing number of T-lymphocytes in the body of an infected person, immunity is significantly weakened, allowing development of a variety of infections that lead to AIDS – a final stage of the disease. The „AIDS“ abbreviation means the Acquired Immune Deficiency Syndrome.