This can be a frustrating problem. Initial things to try would be to make sure you both wash up before sex and you washh up after sex. The cleaner things are, the less likely there is chance for an infection. Other things, non sex related that may put you at risk for yeast infections include hot tub use, sauna use, tight jeans or underwear. limiting these items can reduce your risk of yeast infection. Safe sex (with a condom) can also reduce incidence.
If you have the same partner every time, he should perhaps see his doctor and be given treatment for yeast infection. He may not have any visible symptoms but treatment can still be beneficial.
As for something more serious, the only medical condition that that should be considered is Diabetes. This can be easily tested for by your doctor.
I Hope that helps.
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