13 month old has had a low grade fever of 100.4-100.9 for the past 3 days. how long does a fever typically last and when should i take her to the doctors? she has no other symptoms except diarrhea one time.

Depends: The fever often depends on the type of germ involved & where it is located.Many infant illnesses are viral & fever will last several days before recovery. As long as baby is taking in enough fluids to wet every 6hrs or better, is playful for much of the day & has no specific symptoms you can let it run its course. If baby can't seem to be playful at all, looks sickly or otherwise spooks you go in.

Most likely viral: Viral diarrheal illnesses may last for 1-2 weeks, but the fever should begin to resolve. Continue to monitor for fever spikes. Provide Pedialyte, bananas, rice, applesauce and yogurt until well. Please see your PCP for follow up.