Prostitution is the business or practice of engaging in sexual activity in exchange for payment. The definition of "sexual activity" varies, and is often defined as an activity requiring physical contact with the customer. The requirement of physical contact also creates the risk of transferring infections. Prostitution is sometimes described as sexual services, commercial sex or, colloquially, hooking. It is sometimes referred to euphemistically as "the world's oldest profession" in the English-speaking world. A person who works in this field is called a prostitute, and sometimes a sex worker, but the words hooker and whore are also sometimes used to describe those who work as prostitutes.