When you apply your makeup, you probably don’t expect to be applying bacteria too. Cosmetics such as foundation, eye shadow, blush and mascara can easily become contaminated with bacteria transferred to the containers through contact with your skin. While it can be impossible to keep all bacteria out, there are things you can do to keep these products in good condition.
Always wash your hands before applying makeup, this is good to preserve the lifespan of the products as well as prevent contamination of the skin. Never apply makeup to dirty skin and wash brushes and sponges often, at least once a week. Don’t share your makeup with anyone, another persons germs can be harmful to you. Never add water or saliva to dried out products, this will definitely cause bacterial contamination. Replace your makeup products often. As a general rule, products should be replaced after the following amount of time.
Mascara 2-4 months
Eye Shadow 1 year
Liquid Foundation and Blush 18 months
Lipstick, Gloss, Powders and Powdered Blush 2 years
Lip and Eye Pencils 2 years as long as you regularly sharpen them to create a new tip.
Here’s to beautiful skin – Kristin Malone
Kristin Malone Spa Therapy's Blog 
This is SO helpful! I always forget to throw away makeup and clean my brushes!