Many rashes are caused by allergies, direct contact with skin irritants, some medications and infections that can be bacterial, fungal or parasitic in nature, according to MedicineNet. Eczema, dermatitis, poison ivy, heat rash and hives are examples of some common rashes.
Eczema, also called atopic dermatitis, causes an itchy, red rash that spreads on cheeks, neck, wrists and the crevices of knees and elbows, explains MedicineNet. It's common in those who suffer from asthma or allergies such as hay fever. Contact dermatitis is caused by direct contact with a skin irritant or poison, such as poison oak, poison ivy or other chemicals. It typically spreads in the area that has made contact with the irritant.
Hives are red bumps that come and go, reports MedicineNet. They tend to appear and then resolve in a matter of hours. Those with hives generally experience short breakouts frequently. Psoriasis is a type of rash that creates scaly eruptions and flaking of the skin, typically on the scalp, knees and elbows. Some viral rashes spread symmetrically across the body. They tend to resolve themselves after a few days or weeks.
Heat rash appears as small bumps or blisters, MedicineNet states. It's caused by hot, humid weather and tends to occur in crevices or folds of the body such as the groin, under the breasts and in knee or elbow crevices. It may also spread along the chest and neck. Heat rash is caused by occlusion of the sweat glands, and the most effective treatment is for one to remain in a cool environment.