I know how you feel I had a dun mare that got west nile this fall she had a foal at side.I had the vet out a couple of times and all I could do is watch from the house it was about 2 week's after my wreck sure makes a guy feel worthless she seemed to get better but in the end she died.
As far as it only happening to the good one's thats not true last year I had a 3 year old paint gelding that was a dink he would bite and kick at you.I had a friend breaking him he tore the heck out of his trailer he was plumb nuts they got him saddled in a round pen and he had a halter and lead rope on him he went to bucking real hard.He stepped on the lead rope and flipped over on himself breaking his neck guess he was just not meant to be rode.