My son has had this horrible crud too, running about 5 days behind me with it. He was starting to perk up from it and, aside from still having the really nasty cough, was beginning to feel almost like himself. Until he wasn't.
A couple of days ago, he started heading downhill again. He just came back from the doctor; the verdict is bronchitis, which earns him antibiotics.
This crud is nasty. I recommend avoiding it, if at all possible.