Sick chicken, any ideas?

[LJ from TO]

My year-and-a-half-year old Buff Orpington lost weight over the winter. Recently, her weight loss has become noticeably more severe. She has also become extremely listless, eating very little, sitting in the same place all day, sleeping in her run rather than in the coop at night. I suspect she has some sort of disease but have no clue as to what it might be...Any ideas/suggestions? She is in a coop with 4 other chickens, none of whom seem to be sick.

thanks.
Lorraine from Toronto