Sometimes the contamination may not be in your makeup but in the applicator or brush you're using. Honestly — when, if ever, was the last time you cleaned that blush brush?
Dr. Wu suggests you clean your brushes and applicators once a week. "Not only does this keep bacteria to a minimum, but your makeup will go on better if the applicator isn't caked with old makeup and oils," she says.
To clean brushes, a gentle face cleanser or even baby shampoo and warm water will work just fine. If you do have a cold sore, pink eye, or some other illness, Dr. Wu says to switch to disposable applicators until you're healthy so that you don't reinfect yourself.