One of my chickens died - possible causes

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I had a barred rock that I've been raising since she was two weeks old (one of two runts I couldn't help but rescue). She recovered from her runtiness really well - very much on her way to being a full sized barred rock. She was about three months old. Two days ago she stopped roosting with her "sister". I brought her some greens and she pecked at them, was moving around ok and drinking. I was concerned though that the other one was avoiding her (and these two have always been together).

Yesterday she was asleep first (when I looked out in the evening the others were just settling down and she was already in her spot - same spot she'd slept in for the last couple nights).
Today I went out to refill the feeder, waterers and move the tractor. She was dead, head still tucked down.
Her eyes were slightly cloudy. She was not in any way emaciated - felt plenty heavy (I mean, for a three month old) and fully feathered. She had plenty of energy even last week.

I know I'm throwing out nearly no details but I'm worried about my others.
I checked her eyes - one is cloudy, gray in the center. Folicles are a little bigger than I'd expect but not "inflamed".
Is there anything I should do for the others? This one's gone (and I'm sad - I let these little girls ride under my shirt on the way home the night I got them) - If there's something to be done I'll do so.

 

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That's not a lot to go on, and I'm not really an expert on these things. Cloudy eyes are normal in dead animals. I have a dependable book of chicken diseases with me, and I'll list off the ones that cause sudden death in 12 week old chickens, and I'll do my best to discount the ones I can.

Infectious Anemia (Probable cause, drawn-in head and depression point to it)
Ulcerative enteritis (Possible)
Malaria (It's possible, but really not likely)
Cholera (Possible, not as likely because you would probably notice diarrhea among all your chickens)
Erysipelas (Possible)
Visceral Gout (Possible. It explains why she went off the roost.)
Influenza (It's possible, but not likely. All your chickens would be sick at once, and you would notice many symptoms.)

OK, that's a lot of choices and not much to go by. You'd have to do a post mortem. You can pay a lab to do it, or do it yourself. Only attempt to do it yourself if you have butchered chickens before and know what their insides look like, and there's always some risk that whatever she has could be transmitted to you with certain diseases. Watch your others for similar symptoms. Try researching these diseases for yourself.

Hope that helps.