Hypereosinophilic syndrome (HES) is a rare condition characterized by an excessive number of eosinophils, a type of white blood cell, in the bloodstream. This increase can lead to inflammation and damage in various organs, including the heart, lungs, and skin. Symptoms may include fever, fatigue, rash, and respiratory issues, depending on which organs are affected. The exact cause of HES is often unknown, but it can be associated with other conditions, such as allergies or autoimmune diseases. Diagnosis typically involves blood tests to measure eosinophil levels and may require additional tests to assess organ involvement. Treatment often includes medications to reduce eosinophil counts and manage symptoms.