Why does our skin break out in a rash with some viral infections like measles or Fifth disease?
These sorts of rashes are technically known as viral exanthems (the word derives from the Greek word “exanthema,” meaning “breaking out”).
The skin responds to infection with a rash for one of three reasons: the infectious agent releases a toxin that causes the rash, the infectious agent damages the skin and causes a rash, or the immune response results in the skin outbreak.
The skin responds in only a few ways to these challenges, although the pattern of the response can vary from virus to virus (bacteria and some other infectious organisms can also trigger a rash).
The response is the body’s attempt to deal with the presence of viral particles that find their way to the epidermis, or skin. In general, the upshot of the immune response is an area of inflammation. Because viruses cause a systemic or body-wide infection, viral rashes often cover much of the body.
Although the basic pathway to the rash is similar among viruses, the specific pattern of the rash can help distinguish the virus involved. For example, Fifth disease, so-named because it was the fifth virus in a series to be identified as causing a rash, produces a “slapped-cheek” ruddy appearance on the face and may cause a lacy, rather flat rash elsewhere on the body.
A measles rash, on the other hand, starts as an eruption of raised or flat spots behind the ears and around the hairline before spreading body-wide.
One thing to recognize is that not every rash is a viral rash or a benign viral rash, although most viral rashes will resolve on their own. Usually, a fever accompanies a viral rash. If a rash develops, you should be aware of the following warning signs that signal a call to your doctor:
- If you suspect you have shingles. This highly uncomfortable rash tends to trace along the nerve routes under the skin but can spread out from those, as well. Starting antivirals within the first 24 hours may ward off a more intense recurrence or a permanent pain syndrome called postherpetic neuralgia.
- If you suspect measles. Infection with this highly contagious virus should be reported immediately.
- What you think is a rash from a severe allergic reaction or a rash that arises coincident with taking a new medication.
- The rash accompanies a high fever, spreads rapidly, and starts to look like purple bruising. This pattern is indicative of meningitis.
- Any rash involving a very high fever, pain, dizziness or fainting, difficulty breathing, or a very young child or that is painful.
- Any rash that you find worrisome, including for reasons of persistence or timing with something such as exposure to infection, a new medication, or new food.
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