Gulliver tells Dapple Grey that doctors "get their Livelihood by attending the Sick [...]," a concept that is completely foreign to the Houyhnhnms because they really do not suffer from diseases. He says that we eat when we are not hungry, drink when we are not thirsty, sleep with a lot of people and contract diseases, and get drunk quite a bit, also causing disease; in short, we make ourselves ill with the terrible way we treat our bodies. Thus, a doctor will force the patient's body to purge whatever is hurting it, either by inducing vomit, bleeding the patient, inducing the evacuation of the bowels, etc., and he describes the medicines as being the worst-tasting stuff imaginable, filled with disgusting ingredients that turn the stomach.
Gulliver also says that doctors have "invented imaginary Cures" for the "imaginary" diseases to which we are subject; such diseases most often affect females. Further, he states that if a doctor has predicted that a patient will die, and then that patient begins to recover, "they know how to approve their Sagacity to the World by a seasonable Dose." In other words, he says that, rather than appear to be wrong, a doctor will instead poison his patient!