Eosinophilia is a condition in which the eosinophil count in the peripheral blood exceeds 5×108/L (500/μL). Hypereosinophilia is an elevation in an individuals circulating blood eosinophil count above 1.5 x 109/L  (i.e. 1,500/μL). The hypereosinophilic syndrome is a sustained elevation in this count above 1.5 x 109/L (i.e. 1,500/μL) that is also associated with evidence of eosinophil-based tissue injury. Eosinophils usually account for less than 7% of the circulating leukocytes. A marked increase in non-blood tissue eosinophil count noticed upon histopathologic examination is diagnostic for tissue eosinophilia. Several causes are known, with the most common being some form of allergic reaction or parasitic infection. Diagnosis of eosinophilia is via a complete blood count (CBC), but diagnostic procedures directed at the underlying cause vary depending on the suspected condition(s). An absolute eosinophil count is not generally needed if the CBC shows marked eosinophilia. The location of the causal factor can be used to classify eosinophilia into two general types: extrinsic, in which the factor lies outside the eosinophil cell lineage; and intrinsic eosinophilia, which denotes etiologies within the eosiniphil cell line. Specific treatments are dictated by the causative condition, though in idiopathic eosinophilia, the disease may be controlled with corticosteroids. Eosinophilia is not a disorder (rather, only a sign) unless it is idiopathic.Informally, blood eosinophil levels are often regarded as mildly elevated at counts of 500–1,500/μL, moderately elevated between 1,500 and 5,000/μL, and severely elevated when greater than 5,000/μL.
A brief trial of corticosteroids can be diagnostic for allergic causes, as the eosinophilia should resolve with suppression of the immune over-response. Neoplastic disorders are diagnosed through the usual methods, such as bone marrow aspiration and biopsy for the leukemias, MRI/CT to look for solid tumors, and tests for serum LDH and other tumor markers. Treatment
Treatment is directed toward the underlying cause. However, in primary eosinophilia, or if the eosinophil count must be lowered, corticosteroids such as prednisone may be used. However, immune suppression, the mechanism of action of corticosteroids, can be fatal in patients with parasitosis. List of causes
Eosinophilia can be idiopathic (primary) or, more commonly, secondary to another disease. In the Western World, allergic or atopic diseases are the most common causes, especially those of the respiratory or integumentary systems. In the developing world, parasites are the most common cause. A parasitic infection of nearly any bodily tissue can cause eosinophilia. Diseases that feature eosinophilia as a sign include:

Allergic disorders
Asthma
Hay fever
Drug allergies
Allergic skin diseasesPemphigus
Dermatitis herpetiformis
IgG4-related disease
Parasitic infections
Addisons disease and stress-induced suppression of adrenal gland function
Some forms of malignancy
Acute lymphoblastic leukemia
Chronic myelogenous leukemia
Eosinophilic leukemia
Clonal eosinophilia
Hodgkin lymphoma
Some forms of non-Hodgkin lymphoma
Lymphocyte-variant hypereosinophilia
Systemic mastocytosis
Systemic autoimmune diseasesSystemic lupus erythematosus
Kimura disease
Eosinophilic granulomatosis with polyangiitis
Eosinophilic fasciitis
Eosinophilic myositis
Eosinophilic myocarditis
Eosinophilic esophagitis
Eosinophilic gastroenteritis
Cholesterol embolism (transiently)
Coccidioidomycosis (Valley fever), a fungal disease prominent in the US Southwest. Human immunodeficiency virus infection
Interstitial nephropathy
Hyperimmunoglobulin E syndrome, an immune disorder characterized by high levels of serum IgE
Idiopathic hypereosinophilic syndrome.