There are at least four types of ringworm, including true ringworm, which can appear almost anywhere on the body, jock itch, a type that attacks fingernails, and a type that attacks the scalp. All are treated with anti-fungals.
Considering the broad means of transmition, sex could do it, but so could borrowing a towel. So, it's not really an STD in that sexual transmission is not the only or even the most common way it gets around.