In general, rashes may be caused by bacteria, viruses, allergens, or preexisting conditions such as eczema, hives, or psoriasis. While there may be many reasons for spontaneous rash, the most common reason is dermatitis, which happens when an allergen or irritant makes contact with your skin and causes an allergic reaction. At its core, an allergic reaction means that your body is overreacting to certain substances called allergens. As your immune system intends to eliminate these allergens, swelling, itchiness, blisters, and other symptoms may take shape. Besides allergic reaction, rashes may be induced by stress factors like age, stress, and excess oil on the skin and body. People that have the condition atopic dermatitis (eczema) and people with asthma and other allergies may be even more prone to rashes. Typically, sudden rashes are not too harmful and will go away with time, however in certain cases they may allude to more serious conditions. If you are worried about whether your rashes are harmful or not, you may want to consult your doctor and see if allergy testing is right for you.