A thermometer reading around 98.6 degrees Fahrenheit (37 degrees Celsius) usually means the body temperature is normal. Not any more, says Julie Parsonnet and her colleagues at Stanford University, School of Medicine.

By analysing massive 6,77,423 human body temperature records, collected during the last 157 years, Parsonnet and her colleagues found that “men born in the early 19th century had temperatures 0.59°C higher than men today” and that “temperature has also decreased in women by 0.32°C since the 1890s”. This means that the onset of fever is at 99.5°F and not at 100°F.

What is normal?

From ancient times, physicians have been using body temperature as a critical vital sign in medical diagnosis. A German physician Carl Reinhold August Wunderlich, practising at Leipzig hospital, collected data on body temperature from everyone whom he could lay his hands on. Ill or healthy, he noted down their body temperature many times a day and for several days.