The Scientist: Philip M. Tierno, Ph.D., professor of microbiology and pathology at New York University School of MedicineThe Answer: In a word, yes. In three words, yes, yes, yes.Women are unique in the underpants-buying-market in that they sometimes try on underwear before buying it. And that means that the underwear you buy might have been previously tried on by somebody else. No store policy can guarantee that every interested shopper kept her undergarments on in the fitting room, or that you’ve truly got a fresh paper liner to work with. (Or that there’s no menstrual blood, which Dr. Tierno once found while testing underwear for pathogens.) And even if no one’s ever put on that particular pair of panties, dozens of people have probably handled it—the factory workers who made and packaged it, store employees who put it out on display, shoppers who’ve pawed at them, contemplated buying them, then put them back. With people come germs, such as staphylococcus, MRSA and norovirus (aka stomach flu), which hangs around in a sick person’s body two days after the vomiting and diarrhea have ceased and she feels well enough to go shopping. Germs can live anywhere from a few minutes on cotton to six months, in the case of staph.It’s also important to wash your underwear to dilute the potentially thousands of chemicals used in the textile manufacturing process. Cotton is sprayed with pesticides in the field, which gets incorporated into the fabric. Then it is often blended with synthetic fibers such as nylon, acrylics, polyesters or Spandex, which alone takes 8,000 chemicals to manufacture. Once the pieces are complete, they may be sprayed with fungicides to keep fungus from growing on them in transport, colored with chemical dyes, treated with fixing agents to keep the color from fading, coated with stain resistors and fire retardants, softened with phthalates, and preshrunk with formaldehyde. All—and each—of these can cause itchy contact dermatitis, irritant dermitis and vulvitis, or swelling of the vulva. When the chemicals cause irritation and keep moisture trapped in an already dank region, it can lead to puritus ani, anal itching.The incidence of deleterious effects from new underwear is low—but so is the amount of effort it takes to avoid those effects all together. One good wash will eliminate the bulk of germs and chemicals on a garment and keep you safe and comfortable. And a second run through the laundry can’t hurt.MORE: Ask a Scientist: Is My Vaginal Discharge Normal?