Dun dun dun!
Now, there are a lot of theories out there why Leon never got infected.
1. He was just super lucky.
2. He's part of the 3% of the population who is naturally immune to the T-virus.
3. The only damage that ever happened to him is the bullet wound. No bites and whatnot.
Well, thing with that is that even though the only true cannon damage he received was a bullet wound, he went MUCKING AROUND IN SEWER WATER right after. Sewage water below Raccoon City! THAT HAD T VIRUS IN ITS WATER SUPPLY!
Now I'm not saying I'm gonna retcon the story so he is one of the 3% of the population thats immune, but I am gonna say that neither Claire or Leon knows anything. Therefore, anything is possible.