Pneumonia

Pneumonia, a disease of the lower airways and is a devastating condition in rabbits.
It may develop after a severe case of snuffles, or even develop with very few preceding
upper respiratory signs.
A rabbit with upper airway infection may show few signs, however with pneumonia is often:

Febrile
Anorexic
Lethargic
Marked dyspnea
And had labored breathing.

A Rabbit with Pneumonia may also:

have a blue-ish tinge to ears, lips and tongue, due to lack of oxygen.

Without intervention and treatment, this disease will prove to be deadly.
A necropsy following a bout with Pneumonia will show inflamed lungs.
There are antibiotics available to treat this disease, but very few have proved successful.
Depending on the type of bacteria or virus. The rabbit should be isolated
in a stress-free environment with good ventilation.