It is very hard to diagnose a skin disease without seeing the lesion. Visual inspection is the major diagnostic tool used in dermatology. Measles (morbilli), rubella (German measles), chicken pox (varicella), erythema infectiosum (fifth disease) and exanthema subitum (three days fiver, sixth disease), are all viral infections with skin manifestations (rash). Beside skin rashes, fever is also present during these conditions. Many allergic skin rashes and scarlatina (bacterial infection) may have similar clinical manifestations. You'll need an examination from a dermatologist or an infectologist to establish a correct diagnosis.