Maybe you've experienced a sweat rash before and just took it as a fact of life and dealt with it, but there are ways around it.
A sweat rash under your arms, behind the knees, in between the legs, under or between the breasts or even in the groin area is an extremely annoying side effect of sweating. This irritating condition is caused by yeast and bacteria that occurs naturally on your skin. Sweat rash typically happens where chafing occurs or when sweat gets trapped between two layers of touching skin (or between skin and clothes). Believe me when I say it is not fun when you get it, but don't freak out, sweat rash is a totally normal occurrence.
You can minimize your risk of sweat rash by always showering immediately after exercising, not wearing brand new workout clothes for a long run and by using a method of chafing prevention like BodyGlide. Try and wear clothes that draw (wick) the sweat away from the body. If you do end up getting a rash, treat it with an anti-fungal lotion or cream (get one combined with hydrocortisone to reduce redness and itching). Also, tea tree oil is a natural anti-fungal agent that you may want to consider, but a lot of people shy away from using it because of its strong smell.