Something very nasty - it's a bacterial disease that horses sometimes get when pastured next to a body of water.
It's relatively easy to treat, but can be fatal in extreme cases. It causes diarrhea, stomach ache, fever and loss of appetite - so, it's basically stomach flu. It can also cause miscarriage in pregnant mares. There are vaccines, but they are not completely effective (as is common with bacterial diseases). PHF is a new world disease and can also affect dogs and cats, but not humans. It is not directly contagious - it's caused by the horse eating the bacteria along with their grass.