How many days should a 3 year old child have a fever before you really worry?

What else? If a 3 yr old has fever it is generally a viral illness, but in order to be certain it would be necessary to know what other symptoms are present. If the baby has localized complaints these are best handled by a family doctor or pediatrician. If not, if fever is around more than 5-7 days, a similar visit would also be appropriate, if only to calm parents concerns.