The main reason why children have fever for no reason but are in good spirits is because they have a viral infection. For example, in the case of common acute upper respiratory tract infections, children are caused by rhinovirus or parainfluenza virus, and this is when children develop fever. Sometimes other symptoms are less obvious, such as no obvious symptoms of nasal congestion, runny nose, sneezing, etc. The child is in good spirits and his or her appetite for food is not affected. For example, if a child is suffering from E.E. rash, the child may have a fever, even over 39°C, but the child is in good spirits and eating milk.