Ruxolitinib in Treating Patients With Hypereosinophilic Syndrome or Primary Eosinophilic Disorders

Inclusion Criteria: * Subject with idiopathic hypereosinophilic syndrome must meet the following: * Has as at least 2 readings with an absolute eosinophil count \>= 1,500/mm\^3 in the preceding 3 months prior to starting ruxolitinib (one reading must be during the screening period). * Dependent, intolerant or refractory to corticosteroids OR has relapsed/refractory disease to other therapy besides corticosteroids. * Symptomatic from his/her disease OR has one or more signs of organ damage (assessed by the investigator as possibly-related to eosinophilia or biopsy-proven). This can include skin, lung, cardiac, central nervous system, liver, or gastrointestinal (GI) involvement, or evidence of symptomatic hepatic or splenic enlargement. * Subject with lymphocyte-variant hypereosinophilia must meet the following * Has at least 2 readings with an absolute eosinophil count \>= 1,500/mm\^3 in the preceding 3 months prior to starting ruxolitinib (one reading must be during the screening period). * Dependent, intolerant or refractory to corticosteroids\* OR has relapsed/refractory disease to other therapy besides corticosteroids. * Symptomatic from his/her disease OR has one or more signs of organ damage (assessed by the investigator as possibly-related to eosinophilia or biopsy-proven). This can include skin, lung, cardiac, central nervous system, liver, or GI involvement, or evidence of symptomatic hepatic or splenic enlargement * Has abnormal T-lymphocyte immuno-p…