Its a condition that as yet has an unknown cause but thought to be to do with a toxin. It causes damage to the nervous system and leads to paralysis of the gut. Unfortunately it is fatal in the majority of cases, although some horses can survive with intense nursing and care. It can start with colic symptoms and horses rapidly lose weight.
It is evil, especially the most common form, Acute. Chronic takes a lot of dedicated nursing and horses can make a full recovery. I have personal experience of Acute, if you want to chat please feel free to PM me.