Eosinophil (EOS) deficiency, also known as eosinopenia, is a condition where the number of eosinophils, a type of white blood cell in the blood, falls below the normal reference range. Mild decreases are generally considered clinically insignificant. However, markedly low eosinophil levels can be associated with conditions such as severe stress, Cushing's syndrome (elevated cortisol levels), or alcohol abuse. While not a cause for concern in most cases, it should be noted that in rare instances, it can be an indicator of more serious health issues like heart failure.