Over the past year, fever thermometers have become omnipresent. Body temperature is taken in front of shops, at airports, even before visiting a museum—often with ultra-modern devices. So-called fever cameras or fever scanners can determine the body temperature even without contact. It might now seem obvious that fevers are connected to a high body temperature—in many cultures, the question “do you have a temperature?” is clear evidence of that—but this hasn’t always been the case.