Par4QC what's the problem with this, exactly?
Most people don't know that normal human body temperature is a range, and that it fluctuates a great deal throughout the course of the day. It is quite normal for people to have temps in the 99.9 to 100.2 or 100.3 range even when they are completely healthy.
Using 100.4 as the definition of fever, therefore, eliminates a bunch of false positives.