Loeffler's syndrome appears to be an allergic reaction in one or more areas of the lung. In most cases, the cause can't be determined. But possible causes include:

• A parasitic worm called Ascaris lumbricoides
• Certain medications

Loeffler's syndrome is characterized by chest X-rays that show abnormal shadows (infiltrates), which resemble pneumonia. But these shadows often move (migrate) — disappearing from one part of the lung and reappearing in another part on subsequent X-rays. Tests also show increased levels of eosinophils in the blood and lungs. Eosinophils are a type of white blood cell associated with allergic conditions.

The main symptom of Loeffler's syndrome is a persistent cough. In most cases, the condition goes away without treatment, typically within a few months. But treatment may require corticosteroids. If the underlying cause is a medication, discontinuing the drug usually resolves the problem.