If you suspect that a rash is caused by a drug side-effect, or an allergy to a chemical product of some kind, then the best way to relieve any associated symptoms (the commonest being itching), is to apply calamine lotion. You should, however, always see your doctor, particularly if the problem could be related to a prescribed medicine.
Differentiating between the various types of rash produced by infectious diseases is never easy - even for doctors. Chickenpox is one of the easiest to recognise, since the spots become watery blisters quite quickly.
A measles rash usually starts at the back of the neck, and consists of small red spots grouped together to form blotches. These eventually appear over the whole of the body.
Many articles taken from 'A word with the doctor', by Dr. John Windsor.