Somebody asked me about the replacement of the term STD (sexually transmitted disease) with the term STI (sexually transmitted infection). A little research suggests that this is becoming a widespread term for two reasons. First, it helps emphasize that many sexually transmitted diseases can be transmitted by those who are infected but do not show clinical signs of the disease. Secondly, the term "infection" carries less of a social stigma than the term "disease."
I'm not clear who initiated this trend but many organizations are now using both STD/STI (eg American Social Health Association) some are sticking with STD (eg the Center for disease Control and Prevention) and some are just using STI (eg the World Health Organization)!
The terms are not synonymous, as Wikipedia points out:
'The term STI — which refers to infection with any germ that can cause an STD, even if the infected person has no symptoms — is a much broader term than STD. The distinction being made, however, is closer to that between a colonization and an infection, rather than between an infection and a disease.'