What are sexually transmitted infections (STIs)?

Sexually Transmitted Infection (STI) is another name for Sexually Transmitted Disease (STD). The name STI is sometimes preferred because there are a few STDs, such as chlamydia, that can infect a person without causing any actual disease (i.e. unpleasant symptoms). Someone without symptoms may not think of themselves as having a disease, but they may still have an infection that needs treating.