If a man has a sexually transmitted infection or STI (an infection that is passed from one person to another by sex), the germs that cause the disease can appear in his semen. If the person that the man has sex with touches the semen, he or she can become infected by the germs and pick up the disease. AIDS, gonorrhea, hepatitis B, herpes and syphillis are examples of STIs. One of the ways for a man to lower the chance of passing on an STI to his sexual partner is to wear a condom on his penis when having sex.