What is the definition or description of: hypereosinophilic syndrome?

Hypereosinophilia: Absolute increase in eosinophil numbers, without another obvious cause resulting in a constellation of symptoms including puritis and allergic type symtoms.

Serious illnesses: This is a whole group of diseases, mostly quite serious, which are hard to stop until one notices that the blood eosinophil count is increased, usually above an absolute count of 1000 or 1500. Churg-Strauss is lethal if missed and there are also forms that affect various organs including the heart valves and the lungs. Good luck