How Can Infections Be Avoided While Suffering From Diabetes?

I ve recently been diagnosed with diabetes. I ve a sore on my knee. It started as a round blister. Now it leaks a yellowish clear liquid. This is the second time this has occurred. The first time my doctor swabbed it and sent it off for typing infections. It came back negative for any type staff infection. At this time my doctor started me on metformin. The sore slowly shrunk and eventually stabbed over and healed. That lasted for about 3 weeks. Now it has come back. What I would like to know, how do I keep it from coming back? Summer is coming soon and I want to be able to swim. Oh, I need to add that I m obese.