@SarahGen-- This is true. In adults, the most common cause is drugs. In children, the most common cause is a viral infection.
I have sensitivity to certain groups of antibiotics, mainly penicillin. I have developed a morbiliform rash twice due to antibiotic treatment. Interestingly, the rash did not show up immediately after starting the drugs in my case. The rash appeared one week later.
It's difficult for us to look at a rash and categorize it though. My doctor told me that I should always get checked out because many rashes mimic one another. There are some chronic conditions that cause rashes similar to morbiliform.
In children, I think it's easier to pinpoint since it's almost always caused by a virus.