"Dirty makeup brushes can wreak havoc on the skin. In addition to collecting product residue, dirt and oil, makeup brushes are a breeding ground for bacteria. This could compromise your complexion — in the form of acne breakouts and rashes — as well as your health. Dirty makeup brushes can cause an infection, such as a fungal infection, E. coli, or a staph infection, which can be very serious.To protect your skin and kill any harmful bacteria that lingers in your makeup brushes, it’s a good idea to wash your brushes every 7 to 10 days." -www.aad.org