She had a fever of unknown origin. After taking a few doses of anti-biotic, she coughed up the hairball to end all hairballs, which leads me to believe that it was causing inflammation and infection. We recently switched her from a wet-and-dry combination to strictly dry food to help her teeth, and I think that may have had something to do with it as well. Plus, her old food was organic, and the new stuff wasn't. Who knows--maybe she's smart enough to turn up her nose at "chicken byproducts."