Since a rabbit has to breathe through its nose, any medical condition that restricts or blocks the nose is life-threatening.  An infection of the nasal passages, also known as bacterial rhinitis, is often due to Pasteurella multocida but may be caused by many other species of bacteria. This infection is commonly known as "snuffles" for the congested breathing sound and watery mucus that drips from the nostrils.  It is part of an upper respiratory disease complex in rabbits which may include sinus infections (sinusitis), eyelid infections (conjunctitivis), ear infections (otitis), and trachea and lower airway infections (tracheobronchitis), and may progress to severe infections of the lung (pneumonia).  Rhinitis that has lasted more than 3 weeks, has returned after medical treatment, or has been resistant to initial medical treatment, is considered chronic.

A rabbit with a crusty nasal discharge (red arrow) from "snuffles". Compare to the nostrils of a healthy rabbit.  (Both rabbits were asleep under anesthesia at the time of the photos.)

Snuffles, Mucoid Discharge
This rabbit has a mucopurulent discharge from its nostrils, a much more obvious case of "snuffles".