Is it possible to catch an std from somebody who doesnt have one?

well, and STI can turn into an STD over time if untreated (if that’s what you’re asking)

Are you talking about Bacterial Vaginosis? That’s about the only example of something that is classified in with the STIs that’s along the lines of what you are talking about that I can think of. BV develops as a result of irritation and unbalanced pH levels, both of which can be caused by sexual intercourse. It can also develop, however, following treatment for a yeast infection.

Honey, this one of your only three questions and they are all related. Sounds like you are scared. You can find all of your answers here.
http://www.cdc.gov/std/default.htm