An armpit is a dark, moist place that's a perfect breeding ground for bacteria and other microbes. Some of these bacteria produce odor as they grow, which is why many people use deodorant. But sometimes both the microbes and that deodorant can work against you. The result is a rash.
Rashes come in all shapes and sizes -- some appear as raised patches of blotchy dry skin, others as itchy red bumps or pimples. Some rashes hurt, others itch like crazy, and others may cause no pain or itching at all [source: Mayo Clinic].
Be assured that an underarm rash isn't usually the result of poor hygiene. In fact, a rash may occur as the result of a product you use to stay odor- and hair-free.