Extreme Hereditary Murphy's Law (EHML) is a condition that Milo Murphy and his father Martin Murphy are affected by. It has been passed down through the Murphy family tree since the term "Murphy's Law" was coined by an ancestor of Milo's, Edward Murphy.
EHML (commonly referred to simply as "Murphy's Law") is a condition the Murphy family is affected by, as well as the main premise of the Milo Murphy's Law series. The basic idea of the hereditary disease is simple: it causes continuous catastrophes among members of the Murphy family. The idea of "bad luck" as seen in the series leads to destructive events and damage. Murphy's Law is shown to have extreme social consequences on whoever has it: most people familiar with the effects of the law will stay away from whoever has it.
As seen in the series, Milo's EHML results in a cyclone of calamities which follows him wherever he goes, following the guidelines of Murphy's Law. This may lend to any kind of consequences, from just slipping, to exaggerated catastrophes like entire buildings being destroyed, things catching fire without reason, exploding trucks, or many other types of catastrophic events.
Murphy's Law is totally unpredictable, which means that the severity of the consequences cannot be judged by simply considering the severity of things that have happened before. This means that there is no rhyme or reason to when someone affected by the law will encounter bad luck. Therefore, there is no way to "plan" for Murphy's Law.
Edward Murphy is credited with coining Murphy's Law: the principle that the worst thing can and will happen, and at the worst possible time. Edward Murphy is Milo's great-great-great-great-granddad. He may have been the first one affected by Murphy's law, even if in a less serious form. In fact, he said that "if there is a wrong way someone can mount an engine on a plane, the person who's going to mount the engine will be the one who mount it in that way". Most likely, the Murphy's law "gene" has been transmitted for generations until Milo finally received it. Not much is known about how is it transmitted and if there is a way to "catch" the effects of Murphy's Law. Curiously, it seems that Murphy's law has only affected males from the Murphy family, due to the fact that Sara Murphy, Milo's older sister, isn't affected by it.