My 18 month old has had a fever for 3 days without other symptoms? What is it?

Its time to call. It is time to call the doc. You did the best you could for 3 days, but your child is still ill. Most illnesses are viral and most of them go away with no treatment. You did not say how high the fever was. Hi fevers on and off for 3 days are still probably viral, but if you are not sure, then you should get somebody with experience to check your child. You did well...Now call your family doc.