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. 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 the afflicted. Despite this, those affected by EHML within the series has a cheerful disposition and a positive outlook.
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, ext.
Murphy's Law is unpredictable, the severity of the consequences cannot be judged by simply considering the severity of things that have happened before and that there is no rhyme or reason to when someone affected will encounter disasters. Therefore, there is no way to "plan" for Murphy's Law.
As seen in the show, being affected by Murphy's Law does not negate the possibility of good luck occurring, which is part of the unpredictability of EHML.
Edward Murphy is credited with coining Murphy's Law in our world. the principle that the worst thing can and will happen, and at the worst possible time. Milo claims that his great-great-great-great-grandad is the original Murphy of Murphy's Law . In "Murphy Family Vacation" a scrapbook is shown with many past Murphy family's vacations, one is dated 1871 the family went to Chicago during the Chicago fire of 1871. Also, there is a photo dated April 1912 of their Titanic voyage. And one in San Francisco dated 1906 (the great S.F. earthquake). As Edward Murphy was born in 1918 and died in 1990, he cannot possibly be the "original Murphy" in the Milo Murphy's Law Universe, in fact in the series characters only talk about an "original Murphy" without naming him. Most likely, the Murphy's law "gene" has been transmitted for generations until Milo finally received it. Nothing has been stated about how is it transmitted or if there is a way to "catch" EHML other than by birth. Furthermore, it appears that EHML only affects males, which is demonstrated by Sara Murphy, Milo's older sister, being thus far unaffected by it.