An STI is simply an infection that has been passed on through either vaginal, anal or oral sex. It can also occur through genital contact, i.e. skin-to-skin or close bodily contact, with someone has already has an infection. It doesn't mean you to have be promiscuous to catch an STI.
Sexually Transmitted Infections, or STIs, used be referred to as STD (Sexually Transmitted Diseases) but they've clearly 'rebranded'.
Both acronyms effectively stand for the same, thing, but not all infections (sexually transmitted or otherwise) manifest themselves with particular symptoms, nor do they necessarily develop into diseases. The STD to STI transformation is also designed to be less 'embarrassing'. Try telling those of us who have to fess-up at the Clap Clinic that! Anyway.....
Here we're going give you the lowdown on the most common STIs in a way you can understand and hopefully prevent!