Fever

98.6

Hey, 98.6, it's good to have you back again
Oh, hey, 98.6, her lovin' is the medicine
That saved me, oh, I love my baby

Is a difference from 98.6 normal?

98.6 is the normal body temperature of humans.  But no single value should ever be taken as normal unless you also see an error range.  98.6 is the temperature in Fahrenheit.  In most of the world that temperature would be given as 37 degrees Celsius  If a Canadian heard that the outside air temperature was 35 degrees, he might consider it a heat wave, not because he is Canadian  but because he uses the Celsius temperature system.

Again 98.6 degrees Fahrenheit is 37 degrees Celsius.  The common claim that human body temperature averages 98.6 degrees Fahrenheit originated with a study by the German doctor Carl Reinhold August Wunderlich, who repeatedly measured the temperatures of 25,000 people in Leipzig in 1851. The error range is not often given but based on the instruments of that time it is assumed to be plus or minus 0.5 degrees Celsius.  That means the range of average normal temperature would have been given as 97.7 (36.5 degree Celsius) to 99.5 ( 37.5 Celsius).

But this was in 1851.  Has normal human body temperature changed in the last 170 years?  A body temperature higher than the outside air temperature is a mammalian defense against bacteria and fungus.  For example, the common intestinal bacteria Helicobacter causes open sores called ulcers in the esophagus, stomach and small intestine and raises affected people's risk of developing gastric cancers. Over the years, Helicobacter infections have become less common in the U.S because of antibacterial treatments.  This has meant that the evolutionary bias against body temperatures less than 98.6 degrees Fahrenheit may no longer apply.  According to a recent study, American men born in the 2000s measure an average 1.06 F (0.58 C) cooler than men born in the early 1800s. Women born in the 2000s measure about 0.58 F (0.32 C) cooler than women born in the 1890s. 

The bottom line is that 98.6 may not be YOUR normal body temperature, which might have fluctuated throughout the day anyway.  A fever is when YOUR body temperature is higher than its normal, not when your body temperature is higher than 98.6.