Friction can also cause skin irritation in the underarms -- this is an area of the body where skin continually rubs against fabric or other skin. Friction, combined with heat and moisture, breaks down the epidermis and can cause inflammation. While this may be uncomfortable enough on its own, it's often complicated when bacteria or fungus colonize among the broken skin and infect it [source: Selden].
One specific type of infection that may develop in this manner is candidiasis, or a yeast infection [source: WebMD]. The preventive measures for friction rashes are the same as for heat rash -- keep your underarms dry and wear light, loose-fitting clothing.