Unprotected sex is a major risk factor for HIV infection because it allows the direct exchange of bodily fluids such as semen, vaginal secretions, and blood, which can contain the HIV virus. Without the barrier protection of condoms, HIV can easily enter the body through mucous membranes or small tears in the genital area. This mode of transmission is responsible for the majority of new HIV infections worldwide. Preventive measures, such as consistent condom use, significantly reduce the risk of HIV transmission by blocking the virus from reaching susceptible cells.

Reference: Harrison’s Principles of Internal Medicine, Jameson et al., 20th Edition.