C.H.
There are measles and a virus called Fifth Disease - but most all of these start on the child's core area (tummy, neck, back, back of neck) rather than the arms. Have you inspected her? Are they multiplying. Have you bathered her to remove anything she's come in contact with? If it doesn't go away but doesn't carry any other symptoms like fever, call your pediatrician and ask what it could be before you wasite your time and his but in case it is important. I love that my daughter's birthing hospital had a line to call for any questions like that. A physician's assistant was there and ready to answer all these questions right then and there, no charge.