An act as seemingly harmless as blowing the excess off your powder brush or your blush compact can start a colony of germs (they migrate from your saliva). Aside from being more careful (though everyone has slips, of course), you should be aware of the signs of infection.
Red skin is likely the first symptom you'll notice, Dr. Torres says. "You might start to get an allergic reaction to the product that may not even be associated with the actual cosmetic but may be a reaction to the potential contaminate that is found in the product," he says.
Other immediate warning signs that things are going wrong include any change in the skin, slight inflammation, or itchiness, Dr. Torres says. If you see any symptoms, it's best to stop using the product immediately. See your doctor for advice on treatment as soon as possible.