A condom is a device used to prevent sexually transmitted infection (STI) and pregnancy during sex. It is a "barrier" method, meaning it works by physically preventing direct contact between the two partners. The word "condom" in common usage generally refers to a male condom, also called an external condom. Typically made of latex, this is a thin flexible sheath that covers the bulk of the penis and captures any semen ejaculated. It is effective at preventing pregnancy and stopping the transmission of certain sexually transmitted viruses, like HIV and the Hepatitis B virus. Although less common, there are also female condoms, also called internal condoms. These are placed inside the vagina or anus.