what is the cause of chicken legs illnesses
This is a question that has lots of possible answers! Certainly, there are diseases that can cause leg problems. Marek's disease will cause paralysis of the legs. Diseases like Mycoplasma synoviae can cause swelling of the hocks and lameness. There are some disease agents that cause arthritis. Bumblefoot is often caused by Staph infections.
There are nutritional problems that can cause leg issues. Tibial dyschondroplasia, rickets, etc. are examples. Riboflavin deficiency can cause curled toes.
There are also genetic problems, injuries, and probably more that I'm not thinking of.
So, it's difficult to know what might be causing leg illness. Sorry I'm not more help!