If you suddenly get a rash, it could be a sign of any number of unexpected things. For example, did you know that about one in 20 adults has a food allergy that only started in adulthood? Whatever the cause of your rash — from the band of a wristwatch to a Snickers bar — it’s worth seeing a doctor.

Most rashes are not life-threatening, but some rashes can indicate something more serious. Some of these signs include that the rash is all over your body or is spreading rapidly. It may also have blisters or be causing you pain. If you have any of these symptoms or if you have a rash and a fever, call a board-certified dermatologist or visit the emergency room right away. You may have an infection.

Whatever the cause of your rash, recognizing the signs is the first step to getting the right treatment. Keep reading to learn more about the six signs you should be concerned about a rash.

1. The rash is all over your body

Imagine you’re on a hike and your calf brushes against some poison ivy. The rash that may result is known as contact dermatitis, or a rash that appears where contact with the irritant occurred, according to http://pathologywatch.com. These rashes often clear up on their own within a couple of weeks.

But when a rash covers the entire body — whether after coming into contact with poison ivy, eating a new food or something else — that could indicate something more concerning, like an infection or allergic reaction.

2. The rash comes on suddenly and spreads rapidly

A sudden, quickly spreading rash could be the result of an allergy, perhaps to a medication.

“Skin reactions, including hives and itching and flushed or pale skin” can be signs of anaphylaxis, “a severe, potentially life-threatening allergic reaction,” according to http://mayoclinic.org.

Call your doctor right away — or if it becomes difficult to breathe, go to the emergency room or call 911.

3. The rash has blisters or open sores

If you feel alarmed at the sight of blisters or open sores in your rash, you’re right to feel that way! A blistering rash can be a sign of an infection, a severe allergic reaction or even pemphigus vulgaris, an autoimmune condition, according to  Divya Jacob, Pharm. D at http://medicinenet.com. Seek medical attention, especially if the blistering rash appears around your eyes, in your mouth or on your genitals.

4. The rash is painful

If the rash is hurting you, you don’t have to live with it! Go see your doctor for an evaluation to get treatment and rule out (or treat) conditions like shingles.

“In people who have had chickenpox, the [shingles] virus is never fully cleared from the body; instead, it remains dormant in the nerve tissues,” said Dr. Melissa Conrad Stöppler at http://medicinenet.com. “When physical or emotional stresses to the body weaken the immune system, the virus reactivates and spreads along the nerve fibers to the particular area of skin supplied by the involved nerve (called a dermatome).”

5. You have a rash and a fever

Fevers can often indicate an infection. When paired with a rash, fevers could be caused by conditions like scarlet fever, measles, mononucleosis and shingles, according to the American Academy of Dermatology at http://aad.org. A fever along with a rash could also be a sign of an allergic reaction. Whatever the cause, it’s a good idea to see your doctor if you have a rash and a fever.

6. The rash becomes infected

“If you have an itchy rash and you scratch it, it may become infected,” according to AAD. “Signs of an infected rash are yellow or green fluid, swelling, crusting, pain, and warmth in the area of the rash, or a red streak coming from the rash.” You will want to see a doctor to take care of this infection before it spreads.

Most simple rashes will improve on their own with time, gentle skin care and avoiding the substance that caused the rash. But some rashes don’t go away on their own and require urgent medical attention. If you get a rash all over your body, it comes on suddenly, has blisters or open sores, is causing you pain or becomes infected and if you have a fever, seek medical attention. It’s always better to be safe than sorry!

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