The appearance of an allergic reaction and viral rash is often similar, but the cause of these rashes is entirely different. A rash is rarely serious, but sometimes it can indicate an underlying illness that requires treatment. If you or your child has an unexplained rash, consult your doctor for an accurate diagnosis.
Hives are an allergic reaction characterized by red raised bumps that range in size. These bumps are itchy and can appear anywhere on the body. Small hives can merge together and create large swollen dry areas of skin or the bumps can migrate from one area of the body to another. Severe allergic reactions can cause open sores to appear. Rashes caused by an allergic reaction usually appear within 48 hours after being exposed to an allergen. A drug rash can cover large area of the body. A rash caused by viral illnesses, however, develops as the person starts feeling better. These rashes may or may not itch.
An allergic reaction is caused by your immune system overreacting to a substance known as an allergen. During the initial contact with an allergen, your body begins producing antibodies to fight the foreign substance. The next time you come in contact with the allergen, chemicals such as histamines are released by your immune system and trigger hives. Common causes of hives are poison ivy, medications, rubber, detergents, fragrances and latex. There are several types of viruses that cause rashes: roseola, which is caused by the human herpes virus 6; chickenpox, which is caused by the varicella zoster virus; and fifth disease, which is caused by human parvovirus B19. Other viruses that cause rashes are the coxsackie virus and rubeola virus.
A rash caused by an allergy is treated with oral antihistamines. Acute hives can last a few minutes to a few weeks. If hives are severe, an oral corticosteroid may be needed to reduce inflammation of the skin. If a medicine causes the rash, it takes a few days to a few weeks after stopping the medication for the allergic reaction to disappear. If your airways are swelling from an allergic reaction, seek immediate medical attention.
There is no treatment for viruses, but you can relieve itchiness of related rashes with antihistamines or cool compresses. If you have a fever or the rash is painful, acetaminophen or ibuprofen can provide relief. Antibiotics are only effective at treating secondary bacterial infections caused by scratching itchy rashes; they do not speed the healing process for a viral illness.
Once you're aware of an allergen, avoid it. Unlike respiratory allergy sufferers, someone with allergic contact dermatitis can usually avoid contact with triggers. If you touch an allergen, immediately wash the area with soap and water. If you're in a grassy area, dress in clothes that cover your skin and stay in the middle of cleared paths as often as possible. Always keep track of medications that have caused an allergic reaction in the past. Use detergents that are free of fragrances and irritating chemicals. If you're unsure of the cause of your allergy, consult an allergist. There are vaccinations for measles and chickenpox, which prevent these viral illnesses. Wash your hands frequently, especially after using the restroom and before eating. Avoid close contact with people who appear ill.