Wares, some colic is preventable, like sand colic, other colic just... isn't. Gas colic for example. Impactions, some horses just will block up no matter what you do. Twisted guts "just happen" at times too. And I have no idea how to spell it but interseception where the bowel telescopes, really really nasty, is usually caused by a tumor. Which... we can't exactly prevent.
I'm lucky in that I've only ever had one of my personal horses go down, and that was a mild gas colic that went away on its own, no vet visit needed. Horse was a wimp and had a very low pain threshold and even so his symptoms were minor. But when you work with horses and are active in the horse world you do have close associations with other people who have quite nasty cases. I count myself lucky to have the opportunity to deal with bad colic without having to fork out for the vet fees, although I think it's probably even more stressful for it to be someone else's horse.
MAKORA THOROUGHBRED SPORTHORSES