An interconnected series of air passages, cavernous organs, and cells that permit the introduction of oxygen, the exchange of gases, and the removal of carbon dioxide from the body as well as, in humans, the production of seech.

The anatomical components involved include the external nose and nasal cavity, the oral cavity and pharynx, paranasal sinuses, larynx, trachea, bronchi, lungs, and thoracic cavity; as well as, the muscles related to their activities or functions.

An illness of the nose, throat, and sinus area of felids which is very similar to the common cold in humans: Olive's Egyptian cat was ill and it was diagnosed by the veterinarian as having a feline upper respiratory infection, a dangerous condition but treatable with medications.

In tabbies, these feline upper respiratory infections, which are common and very contagious, are more common in areas associated with overcrowding and poor sanitation, for example in catteries and rescue shelters and also in outdoor feral cat populations (a feral cat or other animal is one that lives in a wild state but was once kept as a pet or lived on a farm).

Several organisms, both bacterial and viral, can cause the feline upper respiratory infection and the two primary viruses usually are feline herpesvirus-1 (FHV) and feline calicivirus (FCV).

Feline Chlamydia, a bacterial infection, can also result in feline upper respiratory tract infections, as well as other organisms which are spread from felid to kitty through eye, nasal, and oral secretions.

Hospital-acquired pneumonia (HAP) or any pneumonia which is contracted by a patient in a hospital at least 48–72 hours after entry: Nosocomial respiratory infections are considered to be the second most common hospital infection (urinary tract infection is the most common) and it accounts for 15–20% of the total infections that are acquired from medical facilities.

Nosocomial respiratory infections are said to be the most common causes of death that are transmitted in intensive care units or infirmaries.

1. Any of the oxygen-carrying substances in the blood and tissues, such as hemoglobin and myoglobin.
2. Any of various colored conjugated proteins; such as, hemoglobin, that occur in living organisms and function in oxygen transfer in cellular respiration.