Kittens can't fight off illnesses due to they are young and haven't developed the part of them
that does fight off disease and illnesses which is why it is very important to spay and neuter your cat.
Because the percentage of a kitten surviving an illness is very small. Most die from the common cold.
Plus kittens need a lot of attention. They need shots for de-worming and everything.
So being a foster parent for cats and kittens isn't just fun and games. There is a lot that doesn't come with the job description.