Sweat rash is extremely common. Unfortunately it can affect anyone at any time but there is no reason why it should cause long-lasting discomfort. This irritating condition is caused by candida, a yeast that occurs naturally on your skin. Trapped perspiration can sometimes upset the natural balance of the skin allowing candida to cause an irritating, itchy and often unsightly red rash. It's difficult to know when it might strike as everyday things such as sweating it out at the gym could potentially lead to this common fungal skin infection. Common areas where this tends to appear are where the skin rubs together: under the arms, backs of the knees, groin and under the breasts.