Do you mean a yeast infection? (A fistula is one of the worst possible complications of GRS - a hole right through the vagina, often into the rectum.)
You CAN get a yeast infection and any other STD the same as a cis woman could. I recommend looking for some guides online to how to have safe sex; they're all over the place and can get pretty detailed.
Yeast infection is a bit of a misnomer anyway, as it's not an infection per se; yeast are a fungus, and it's natural to have *some* of them live in the vagina, but sometimes they overgrow and become a problem. You can get it from oral if the guy has a yeast infection in his mouth, but it's more likely from antibiotics or too-tight panties (yeast like a warm moist environment, so if you make it extra confined in there, they'll go wild).