The following is the single most useful piece of information I've found about flu, because it can occasionally can lead to pneumonia and this is a warning sign.
Quote:
New York Times, Denise Grady, Sept. 3, 2009.
http://www.nytimes.com/2009/09/04/healt ... .html?_r=1 “ . . . In children without chronic health problems, it is a warning sign if they seem to recover from the flu but then relapse with a high fever, Dr. Frieden said. The relapse may be bacterial pneumonia, . . . ”
And presumably this is the case for adults, too.
========
As far as your specific question, well, you don't want your fever to be dangerously high, but I can see how a medium fever could be useful. Maybe other people have more detailed information.