Fever and rash in 18 year old any ideas?

Go to doctor: Go to the hospital! this could be a life threatening infection or something not too worrisome. Rule of thumb: fever + rash = seek medical attention, especially in a teenager.

Probably viral: The most common cause of a rash accompanied by a fever is a viral illness; but there are many other possibilities. Teens don't tend to run a fever unless something fairly significant is going on, so you should have him or her evaluated by your doctor or a dermatologist.