Generally over 105. I have always been taught that any temperature over 105 requires being examined, no matter what. Under 6-8 weeks of age anything over 100.5 is considered emergent. But this is the fever only. You need to look at the whole package. That incudes pain, irritability, lethargy, breathing trouble, dehydration and other symptoms. So if concerned, call the pediatrician.
No fix cutoff. What makes a fever worrisome regardless of the number depends on age of child, and what else is going on the the child. For example, a sleepy one month old infant with 100.5f temperature is more worrisome than an alert, interactive 5 year old with temperature of 102f. So, if you are worried about your child's illness, call his/her doctor, even if there's no fever.
It depends! Fever is one way the body fights infection. It's not all bad! There is no.