Caitlin Callahan once grabbed a tube of mascara she thought was brand new and applied it to her lashes. Oops: it was actually an old tube she’d forgotten to toss. Within hours, she felt like she had hot sand in her eye and had to dash to a walk-in clinic for antibiotics.

Callahan, a senior artist with the M.A.C. Pro team in Vancouver, tells this story to her clients to illustrate a little-known fact: Dirt and bacteria from hands, faces and the environment can get into your makeup, applicators, bags and drawers—and using makeup contaminated with staphylococcus or E. coli can make you sick.