 A condom is a sheath @-@ shaped barrier device that may be used during sexual intercourse to reduce the probability of pregnancy and decrease the risk of sexually transmitted infections ( STIs ) such as HIV / AIDS . It is rolled onto an erect penis before intercourse and blocks <unk> semen from entering the body of a sexual partner . Condoms are also used during <unk> and for collection of semen for use in infertility treatment . In the modern age , condoms are most often made from latex , but some are made from other materials such as polyurethane , polyisoprene , or lamb intestine . A female condom is also available , often made of <unk> rubber . 
