I like to ask Dr. Sears:
"WHAT TEMPERATURE CONSTITUTES A FEVER?
Normal temperature – 97 to 99 degrees (36 to 37.2 Celcius).
Low-grade fever – 99 to 100.9 degrees (37.3 to 38.3 Celcius).
Common fever – 101 to 103.5 degrees (38.4 to 39.7 Celcius).
High fever – any fever over 103.6 degrees (39.8 Celcius)."
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If your child of any age has one or more of the following symptoms, you should probably call your doctor right away:
High fevers of 104 (40 Celcius) or higher that don't come down to 101 or 102 (38.3 to 38.9 Celcius) with the treatment measures below.
Lethargy – this means more than your child just isn't acting right or laying quietly in your arms. Lethargy actually refers to your child being limp, lifeless, unresponsive, or won't make eye contact.
Irritability - this means more than just fussiness. A truly irritable child will cry for hours with minimal verbal interaction, and is almost impossible to console.
Meningitis – symptoms are high fever, stiff neck or pain in the back of the neck, vomiting, headache, bright light hurts the eyes. Before paging your doctor, you should also look up any other symptoms your child has, such as cough, vomiting, rash, etc and read those guidelines to determine your best course of action
...
ABOVE ALL, IF YOU HAVE A "GUT FEELING" THAT YOUR CHILD IS SERIOUSLY ILL; YOU SHOULD CONTACT YOUR DOCTOR RIGHT AWAY."
http://www.askdrsears.com/html/8/t082100.asp