The concept of there being a normal body temperature is somewhat misleading. It is generally accepted that normal body temperature ranges between 36.1C (97F) to 37.2C (99F). although it depends on:
* The person
* Their age
* What they've been doing ( as in activity)
* The time of day
* Which part of the body you take the temperature from.
The concept that normal body temperature is 37C (98.6F) stems from work performed by German physician called Dr Carl Wunderlich. He is credited with taking temperature readings from thousands of patients, which led him to propose that 37C was normal body temperature.
Research from Winthrop University in the US published in 2006 found that older people have lower temperatures and that, even when ill, their bodies may never reach temperatures that would be regarded as fever.
As others have mentioned the gold standard for obtaining a core temperature the true reflection of someone actual temperature is by taking a rectal temperature as this closely relates to the actual body's temperature.
Where I work ( ITU) we use rectal temperature probes for motoring our patients temperatures, however with the advent of catheters with a temperature port we are increasing using them.
A patient temperature is only one parameter used in determining if someone has an infection, others include white blood cell count ( WBC) , Erythrocyte Sedimentation Rate (ESR), C reactive protein(CRP) and actual physical exam , wound observation ( colour, heat, smell). etc