It's totally natural for parents to be concerned when there's no official diagnosis for their child's illness. But your doctor probably felt pretty confident that your child looked good when he examined him and thought he could wait and see how his illness evolves over the next three to four days. It could turn out to be roseola, a virus that causes a high fever, but can only be firmly diagnosed after a rash appears a few days later. Your child's demeanor is a good clue as well -- at this age, there's a big difference between being a little fussy or totally inconsolable (much more worrisome). If you still can't stand the uncertainty, call your doctor tomorrow and ask for your child's blood count. If his white blood cell count is normal, you can rest assured that it's just a virus. Remember, it's always okay to ask your doctor to do a test.