How long would you have to have an STD to start having feritility problems?

I have just found out my partner had an affair starting February 08 for three months, he told me he did not always use protection, so I need to go for an STD check up. We are trying to concieve and I just wondered if I did have an infection would it had of caused fertility problems already and how long does it usual take to cause any problems? Helpful and facts only please! All answers appreciated!