This is a more serious skin condition that is not caused by sweat per se, but it does aggravate it. If you have eczema, exercise can make it worse because when you sweat, the moisture evaporates to cool you down and causes your skin to dry out. This leaves a salty residue that irritates it further. It can even make you feel itchy during your workout.
Yes, I am going there. When you sweat, sensitive areas such as your breasts, groin, and underarms are more likely to come into contact with bacteria since they have folds. They are the perfect condition for harboring candida yeasts, which can develop into a yeast infection unfortunately.