A fever gets its name from the Latin word “Forveo,” meaning to burn or be hot, because it is a fervor or heat affecting the body. The Greeks call it _pyretos_, meaning to be inflamed or taken with fever. Sometimes it is called _pyros hexis_, meaning a fiery habit or fiery disposition of the body. Hippocrates in the first book of Epidemics, Commentary 3, text 18, calls it simply “fire itself.” **Definition** A fever is an unnatural heat kindled in the heart as its primary source, which directly impairs our bodily functions. This heat spreads throughout the entire body through the blood and vital spirits via the veins and arteries. **Types of Heat** All heat is either natural or acquired. Natural heat is either innate and fixed, or elementary and fluid. A fever cannot consist of either of these natural heats, because: - The innate heat is nourished by the body’s original moisture, which has a heavenly origin and cannot be restored once depleted - The elemental heat helps and cherishes the innate heat, assisting it in digesting and assimilating nourishment that converts into our bodily substance Physicians call this natural heat “influent” because it flows with the spirits and blood from the heart through the veins and arteries to all parts of the body. Feverish heat, therefore, is acquired heat, as Galen states in his Commentary on the sixth book of Hippocrates’ Epidemics, text 28.