Religion

When we talk about liver function, or creatinine clearance, or BNP levels, or left ventricular ejection fractions, or blood pressure, or whatnot we have an actual range which healthy people demonstrate and the conversation is about "within normal limits." And different labs and hospitals actually sometimes have slightly different ranges they consider "normal" on a lot of these.

But "normal" temperature doesn't have a range, unless you consider 37 degrees to 37 degrees to be a range. They didn't arbitrarily pick that out of thin air, that's the normal temperature they've established by checking the temperatures of billions of people at multiple points in their lives. "Afebrile" is 37 degrees. We've got a pretty good grip on the math for this one, backed up by more formal and informal experiments than anything this side of gravity. Some rare individuals do run on a slightly different temperature, but some people also have three kidneys. It's not necessarily bad, but it is definitely not normal. Normal is two kidneys and 37 degrees.

When you go to the hospital, your doctor will establish whether you are febrile or afebrile because it's part of the decision-making flowchart. If the answer is yes, then they worry about how much of a fever you have, why, and what to do about it. But it is impossible to make it out of a hospital visit without either "febrile" or "afebrile" appearing in your record. Go ahead. Try it. It can't be done.

There is indeed a spectrum for having a fever, but the spectrum starts at "no." It's kind of like the spectrum for how much cocaine you have in your system, or how broken is your leg. Or how religious you are. Zero appears on all of these scales. If you don't have a religion, or a fever, or cocaine, or a broken leg, or a Playstation, or whatever, then you don't have them. We have terms for people without certain things, and those things are chosen quite arbitrarily, but people without a religion are called "atheist" just the same way people without fevers are called "afebrile."