A lot of new HIV infections are passed on by men who believe that they are HIV-negative. When you get close to someone, it's easy to develop a certain level of trust and have sex without a condom. In this way, many men take risks without knowing it. People who are not aware that they have been infected with HIV unknowingly pass the virus on to others and become part of a chain reaction that could have been prevented.
In the first weeks in particular HIV is so infectious that one single unprotected encounter could result in an infection.