Why? Well, yeast thrives on moisture — so if you trap moisture in your fussy silk or mostly nylon-blend underwear, you’re creating a prime breeding ground for an infection (ouch). “Of course, if you’re the kind of person who never has issues with vaginal irritation and infections, you can wear whatever you want,” Dr. Dardik says. But if you’re prone to these problems, opting for underwear made up of mostly cotton — or, at the very least, has a 100% cotton lining — is your best bet.