Stray and ILL Kitten

A stray mother cat had kittens. (I live in an area that seems to be a dumping ground for horrible humans or people who are running off and leaving pets. This isn't the first time I have dealt with strays.) Not long after catching them, I noticed that one was smaller and ill. I took him to the vet and they gave him fluids and parasite meds and said he would probably die and gave me tips to keep him comfortable. They had no explanation. They said it was a defect or unknown cause.  I've never seen a kitten this sick.  He died. The other kittens and the mother were healthy. The kitten had obviously been fine the first few weeks due to its size.What could have happened?

There are so many things that could go wrong with kittens born outside.  Fleas can kill a kitten fast, as can any number of other parasites.  He may have had a birth defect that killed him slowly.

Blessings on you for offering him love and attention in his short life.