Rashes can be caused by many things, including exposure to certain plants (poison ivy, for example), allergic reactions to a medication or a food. Rashes can also be caused by an illness such as measles or chickenpox. Eczema and hives, both of which are related to allergies, are two of the most common types of skin rashes. If your skin condition is the result of an allergy, an allergist can diagnose and treat your condition, so you can live life to the fullest.