He was fine when we left to go to the store yesterday evening. When my husband found him later that night, he was lying on his side with his eyes open and he was already gone. There was nothing wrong with him that anyone knew about, he was still a kitten, but it appears he had some sort of hidden condition that no one ever caught until it was too late.
We have the worst luck with cats that adore our children and vise-versa. The last one was another indoor cat that bolted out the door and was struck by a car. Before that, we had a special needs kitty that died during a seizure. My grandmother's sixteen-year-old cat passed away this summer due to infection and he was like a brother to me since I grew up with him.
Though we still have one cat, she is a stray snowshoe that we found while looking for our boy when he disappeared earlier this year. She has a neurological condition and I'm concerned that she may not make it out of kittenhood. If she does, I still don't expect her to live a long life, but hopefully not a painful one.