The whole phlegmatic/choleric/etc. nonsense was lifted entirely from Galen of Pergamon, who applied Hippocrates' notion of the four humors in medicine to variations of personality and behavior. So, if it looks like we're looking at educational advice from the kind of people who would prescribe bloodletting for the flu, that's because we are.
Just so that there is no confusion here, Dude does not mean to assert that bloodletting is *not* a very appropriate treatment for certain medical conditions, such as hemochromatosis
He is simply clarifying that bloodletting is *not* an appropriate treatment for viral conditions and should not be used as an educational tool.
