Churg-Strauss syndrome (CSS) is an extremely rare disease – there are only 2 to 5 new cases per year per 1 million people. CSS results from inflammation that occurs in certain types of cells in blood or in tissues. This inflammation causes injury to organ systems – the most commonly involved are the lungs, nose, sinuses, skin, joints, nerves, intestinal tract, heart, and kidneys. A main feature of CSS is that all patients have had a history of asthma and/or allergies.

Almost all people with CSS have increased numbers of "allergic type" blood cells called eosinophils. Eosinophils are a type of white blood cell. Normally, eosinophils make up 5% or less of the total white blood cell count. In CSS, eosinophils usually make up more than 10% of the total white blood cell count. In addition, most biopsies contain clusters of cells called "granulomas" that may or may not involve blood vessels. The granulomas often include eosinophils and are therefore called "allergic granulomas." In fact, another name for CSS is allergic granulomatosis. Some of the most serious manifestations of CSS are related to the presence of vasculitis.