How does Makeup become contaminated you ask? It's literally due to bacteria naturally found on the skin but have overgrown in your makeup over time due to constant touching and a breakdown of the antibacterial agent over time. Over 50% of samples tested from old makeup contained bacteria such as Staphylococcus & Streptococcus. To read more about makeup contamination/expiration you can check out a previous post here:
When you use contaminated makeup products it can lead to acne called ACNE COSMETICA. This occurs due to a blockage of pores and bacterial overgrowth. More severe cases could include boils and abscesses on your skin due to infection. If you're currently battling acne or skin issues which do not seem to respond to treatment, check your makeup.
This is why it's so important to keep your makeup clean, here are some tips: