Mass Die Off

A Mass Die Off is the whole scale death of a large number of creatures, usually with similar physiology if not outright members of the same species. MDO's are unnerving because they always bring with them the threat of biological or chemical contamination, but many MDO's, even well reported and investigated MDO's, remain unsolved to this day.
References:
http://www.end-times-prophecy.org/animal-deaths-birds-fish-end-times.html
http://science.nationalgeographic.com/science/prehistoric-world/mass-extinction/
http://listverse.com/2013/11/09/10-devastating-mass-animal-die-offs-from-2013/