Eosinophilic granulomatosis with polyangiitis (EGPA) is a rare immune system disorder that causes asthma, an excessive number of eosinophils (a type of white blood cell) in the blood, and the inflammation of blood vessels, or vasculitis. In order to properly treat EGPA, it is critical that the level of disease activity can be determined over the course of the disease. The purpose of this study is to determine new biological markers, or biomarkers, that may be used to assess the severity of this disease in people with EGPA.