I'm so sorry to hear you lost so many precious ones. It sounds very much like panleuk/distemper as Moosmom mentioned. I've had it hit here a few times, and it's a devastating virus. Symptoms all sound to me like that was the case. I take in so many rescues year after year, that I've decided that better safe than sorry, and I vaccinate all that are old enough to be done. But sometimes vaccines can't be used if the kitties are sick already with URI. I do my best to isolate newcomers and cover cages, and wash between each new group, but there's times when even that doesn't work, and the virus from hell hits. RIP all the little ones that are now angels over the Bridge.