Welcome to BYC. Do you live in the US? You have to use good biosecurity methods with a sick chicken (handwashing, change shoe, clothes, cover hair, and wash utensils and feeders separately /disinfect) as well as separating the sick birds far away. Your symptoms sound like coccidiosis. Most hens should be immune to coccidiosis germs on their property, unless they are new to the dirt or a new bird has recently been added. Treatment for cocci is Corid/amprollium for 5 days in the water. On the neck position, do you mean huddling or puffing up, or do you mean twisting the neck? Here is a link to read:
http://www.the-chicken-chick.com/2012/12/coccidiosis-what-backyard-chicken.html
This is a picture of huddling or puffing up from this link:
http://www.chickenvet.co.uk/health-and-common-diseases/coccidiosis/index.aspx