Madness (or Insanity) is a word people use to describe different diseases of the mind. Usually, such people need to be cared for. It can also refer to anger. But some people question what mental health truly is.
Madness has proven, in some cases, to be caused by highly infectious virus, related to Rabies. Outbreaks of this particular virus have been only been reported in isolated areas of West Africa, and evidence of the claims have been discredited by some as "anecdotal."
People who are mad (or insane) usually have a hard time telling right from wrong. In most countries, being able to tell if something is wrong is the basis for criminal law. Mad people can therefore not commit crimes in such countries.