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FKS = Fading Kitten Syndrome (not KDS, my mistake)

Apparently their immune systems are very weak early on. If the mother abandons them early (as happened with our three) the incidence rate goes up. I think we could have saved our guy getting him to the vet a couple of days earlier, but I couldn’t get approval until too late. A pity. Sweetest critter I’ve ever seen. If I ever foster kittens again I will insist on being able to take them to the vet on my say so, and will pay if necessary. I won’t watch one fail like that again while my hands are tied.

http://www.catster.com/lifestyle/ask-a-vet-foster-kittens-die-fading-kitten-syndrome-cat-health

https://www.petmd.com/cat/conditions/fading-kitten-syndrome-symptoms-and-treatment-options