3 year old 104 temp. no other symptoms other than just not feeling good.

Any rash?: if there is a rash then it is likely a virus. there are amny viruses going around this time of year. any other symptoms? tummy pain, nausea, vomiting. try tylenol (acetaminophen) or motrin. if it still persists and additional symptoms arise see your pediatrician.

Common issue: Fever, especially when high, is a common first phase of a new viral illness. It can also occur with bacteria, but viral illnesses often waver up and down between high and low but will almost always start to involve other symptoms like nausea, vomiting, cramps, muscle pain,etc. If taking fluids well, the best thing to do is focus on hydration and wait. If the symptoms become alarming, see the doc