Is my flock carrying some kind of disease or parasite?

[musicalchicken]

I have recently lost one of my three year old chickens. She was a very healthy chicken until one day I noticed she was very skinny. She started sleeping a lot, had no appetite, and had very runny, yellow poop. One of my two year old chickens is beginning to show some of the same signs, but not all. She has runny yellow poop and is still eating but not like the others. Now I am starting to wonder if my flock is carrying a parasite or a disease of some kind. I try to keep it as clean as possible in the coop. I also noticed my other chickens are some what skinny. I feed them pellets, sun flower seeds, bread, and fruits and veggies pretty much every day. Maybe someone can help me out and give me an idea of what might be wrong. Also, how can I fatten them up a bit?
Thanks in advance.

 

[keesmom]

Have you wormed them? Or checked for lice and mites?

 

[musicalchicken]

No, I have not done any of those things but i will check tomorrow. I have never wormed my chickens before. How do you do that?