A “rash” is a general term for inflammation of the skin. There are many causes of a rash, including, but not limited to, allergic or irritant dermatitis, autoimmune disease, infections, medications, and rarely cancer of the skin.
Allergic contact dermatitis is a skin condition that arises after contact with a chemical from which you have developed an allergy. Common allergens are found in makeup, soaps, detergents, colognes, various plants such as poison ivy, or other chemical that may be found at home or in the workplace. It is sometimes necessary to do allergy testing to determine which chemical may be causing the rash.
Irritant contact dermatitis is similar in appearance to allergic contact dermatitis, although the etiology is different. It is caused by contact to various chemicals that we use everyday for cosmetic or cleaning purposes, as well as from chemicals in the workplace. These chemicals cause the skin to become inflamed from direct irritation, but not due to an allergic reaction.
Autoimmune disease of the skin is caused by an abnormal response by the immune system. These diseases are often exacerbated by sun exposure. Some examples of autoimmune disease include Psoriasis and Lupus. It is important to evaluate these diseases to ensure that other organs are not involved.
Infection of the skin can cause a rash as well. Bacteria, fungi, and viruses can all affect the skin. Bacterial diseases generally appear as pustules, tender crusted areas, or in ring like patches (Lyme disease).
Fungal diseases such a ringworm or athletes foot are also very common.
Viral diseases can present as rashes, either directly such a herpes, chicken pox, and shingles, or indirectly as a rash appearing during a viral illness, such as roseola.
Medications are a frequent cause of rash. Basically, any medication can cause a rash to develop in any person. Common medications that cause a rash include antibiotics, and medications for high blood pressure and cholesterol.
Rarely, cancer of the skin can present as a skin rash. Superficial skin cancers, such as basal cell or squamous cell carcinoma can present as scaly pink patches. Lymphoma of the skin can present as a scaly rash on the body.
Many of these rashes look the same to the untrained eye, therefore it is important to consult a dermatologist, a specialist in this area, when you have a skin condition that is not going away.