what do know about HIV - Infection with HIV occurs by the transfer of blood, semen, vaginal fluid, Cowper's fluid or breast milk. Within these body fluids HIV is present as both free virus particles and virus within infected immune cells. The three major routes of transmission are unprotected sexual intercourse, contaminated needles and transmission from an infected mother to her baby at birth or through breast milk. Screening of blood products for HIV in the developed world has largely eliminated transmission through blood transfusions or infected blood products in these countries.