Previously called bronchiolitis obliterans with organizing pneumonia, cryptogenic organizing pneumonia (COP) is a rare lung condition in which the small airways (bronchioles), the tiny air-exchange sacs (alveoli) and the walls of small bronchi become inflamed and plugged with connective tissue. The condition is called "cryptogenic" because the cause is unknown.

Most people who have COP experience a persistent nonproductive cough and — depending on how much of the lung is affected — may have shortness of breath with exertion, a low-grade fever and a general unwell feeling.

Before diagnosing COP, your doctor will want to rule out other possible causes of pneumonia, such as:

• Bacterial, viral or fungal infections
• Exposure to drugs or medications such as cocaine, gold salts, and some antibiotics and anti-seizure medications
• Inflammatory disorders such as lupus, rheumatoid arthritis or scleroderma
• Bone marrow, lung, kidney and stem cell transplants
• Certain types of chemotherapy or radiation to the chest

Most people with COP recover after weeks or months of treatment with corticosteroids. In some cases, however, COP can progress despite treatment.