Sudden death

[chooks4]

My 35/40 week old Blue Marian has died . I have no idea why , the night before she died I was checking on them and I noticed she was sitting by the water as if she was going to lay, her whole back end was slanted . This morning I found her dead in the nesting box very stiff and her eyes closed . She could not have been dead long as she had layer her egg and it was about 10 o clock . I don't know how she died ? Could it be a disease if so how do I stop my other 3 chickens from catching it?!

 

[canadian flock]

If she was with the other chickens, they likely already have it if it is transmissible chicken to chicken. Does she have any other injuries or had any other sicknesses in the past?