If he has had a mumps shot, chances are it isn't that. It could be roseola, which is fairly common in children of this age.
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Fifths disease is another common virus:
Hope that helps!
Your son looks like has Roseola not Mumps.You are not suppose to see rash in mumps. Once the fever subsides, a rash appears. The rash consists of many small pink spots or patches. These spots are generally flat, but some may be raised. The rash usually starts on the chest, back and abdomen and then spreads to the neck and arms. It may or may not reach the legs and face. To read more please click the link just below:
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