One of my four month old chickens was fine this morning (saw her eating, drinking, scratching around). I went out this evening to check their water and she was lying on her side in the run, completely unresponsive. While I was watching her, she vomited (foam?). I tried to pick her up and she fell over. My uncle examined her, no signs of injury, everything appeared normal, no weight loss. She died about an hour later. The other two appear to be acting normally, I saw them eating and drinking while I was sitting with my poor girl. I looked at their poop (we lost one chicken to suspected coccidiosis when they were about ten weeks old, treated them all with Corid at the time). It seemed watery, but no blood or strange colors. Any ideas on what would cause such a sudden decline? Is there anything I can do for the rest other than watch and wait?