In Revelation 8:10-11 the bible talks about a "star" named "Wormwood" that will poison a third of all fresh water in the world. In historical terms, an asteroid or meteorite was sometimes referred to as a "star", especially if it fell from Heaven. The term "Wormwood" is taken from the genus Artemisia, a series of ornamental plants known both for their beautiful decorative appeal and poisonous properties. It's not known the means by which this judgment will be carried out. But since the symptoms of the poisoning caused by this event are very similar to those caused by thujone, the poison that makes Wormwood trees so deadly. Thus why it was likely used by John to describe this plague.
However, the actual plague doesn't need to originate terrestrially. Nasa has proven through extensive research that thujone also has the potential to exist within the bodies of large asteroids. As such the "star", as spoken of in Revelation, may indeed be a large asteroid that explodes high in the atmosphere of Earth resulting in the poisoning of large swaths of water and land in the process. What will make this plague so devastating is that it will happen at a time when people are most desperate for water and will drink it, either knowing it is poisoned, or perhaps even unknowingly, and die from it. There is no mention of how many die, but the idea of "many" indicates that the death toll will be quite significant, if not outright staggering.
Another question that comes up about this event is whether people will be aware that the waters in their area have become poisoned. Given the amount of devastation that will have occurred up to this point, it's unlikely that anyone will be notified of this right away, if at all. In fact, it'd be a bit surprising if any part of society in general was still functioning by this point. I suspect it will be, as the AntiChrist is seen by the entire world around the midpoint of the Tribulation, of which this event comes well before then, but that doesn't mean that the citizens of the world will be kept aprised of this development as such information is certain to cause widespread panic.
(*Do you have a better description, or did you find an error? Please let me know.