I'm terribly sorry to hear about your bird... loosing them is always so sad. Mites can lead to a chicken's death because their pressence drops the immune system of the bird they inhabit which makes them die of other things that they catch. To prevent this from happening more I would treat all of them, regardless of if any appear to have them, just in case. Start checking them monthly for mites, respiratory problems, and bumble foot. These simple and quick check-ups can make a big difference if you catch them early on.
For mites:Go out at night and examine the vents of your birds using a flash-light. If you see scabs, inflammation, or the bugs themselves, then your birds have mites. It's one of those "where there's one there's many" kind of a thing, so treat all of them intensively even if only one has the bugs. Diligence is the only way to get rid of them, so be persistent!
For respiratory: look at the eyes of your bird carefully... Look for bubbles, cloudiness, ect. Look for discharge from nostrils and eyes. Also make sure there is no swelling in the face.
For Bumbblefoot: Examine the pads of their feet... You should make sure they have no scabs or swelling of any kind.
I hope this helps!!! Again, sorry about your chicken. Starting these checks monthly dose make random deaths like this MUCH less frequent.
-G
P.S. If your birds have mites now I can help... just let me know
