Scientists are looking for the LUCA. Don't bother checking under the couch, it's probably long gone. The LUCA stands for Last Universal Common Ancestor. That is, the last lifeform that all present lifeforms on earth can trace their heritage to. Scientists had thought that the LUCA would have to be something barely definable as a lifeform, but recent findings have given some food for thought. Scientists have found some qualities that are shared by all earth lifeforms that are actually fairly advanced. So while they might not find the real LUCA it seems they are piecing together a picture of what the LUCA must have been like.
My story would take this to the next level. Some smarty pants scientists has figured out the perfect model of the LUCA and then tries to create as a synthetic lifeform. Nothing good can come of this, but that's movie science for you. In my story the LUCA because it shares something with every lifeform on Earth it is also able to infect every lifeform on Earth. No other disease can do that. Now of course, since this is a movie the LUCA moves out and all hell comes to play. Most stories about diseases and such are naturally centered on humans getting infected, but trust me you'll get some attention if say half of every crop planted is lost in say two weeks.
Of course there are questions that would have to be considered. Would it follow standard models of diseases, or what? What does the LUCA do when not infecting its descendants? Would it be more devastating to higher lifeforms that share less in common with the LUCA or lower closer relatives? A world without soil bacteria, for example, wouldn't be a pretty place.
Anyway, that's my mad idea of the day.