Suggest Treatment For Itchy Rash On Body

I had what I thought was measles, however the blood test that they did on me for measles showed that I have an immunity to measles and they believe I have a virile infection of some kind. I had a rash on all parts of my body even the bottom of my feet and palms of my hands and I itched bad. But no fever or sore throat. What steps should I take next to assure me of what infection I have or had. The red rash is diminishing, but I am not satisfied not knowing what it is. I am 72 years old and have been taking the same meds for 10 years with no problem. Thank you