Poor dental hygiene:
Recall the scene in the movie Gladiator when slaves were selected for fighting by checking the slave's gums to verify if he was a good purchase.
If your gums are bleeding due to infrequent flossing and improper brushing, your body is constantly making C-reactive proteins which means your body is in a chronic state of inflammation. This robs protein that could be used to synthesize new muscle tissue to fight off the infection. Throughout the ages, it has been known that dental health is a reflection of overall health.
This is why over the years I have observed that athletes always go through training plateaus after visiting a dental hygienist, since teeth cleaning can aggravate inflamed gums. Look around your gym, the easy gainers always have perfect teeth.