What to do to keep my flock healthy?

I got 12 new chicks this past March and everyone has been doing fine... until today. I went out this morning and found one of the white hens just laying there in the coop. She would eat a bite and drink a little but could not stand. I brought her inside to watch her. She had loose bowel movements for about 3 hours and then just died. She looked so sad... almost like she knew she was gonna die. The others look normal but are not eating well at all and they have stopped laying eggs. Do you think they are all getting sick? What should I do?