Parasitic Infection: This is often the most common cause of elevated eosinophils, and monocytes can also be elevated in response to chronic infection or inflammation. Parasites such as hookworms, roundworms, and tapeworms are typical culprits.

Allergic Reactions: Allergies can cause an increase in eosinophils, and if the reaction is severe or chronic, monocytes may also be elevated. Common allergens include pollen, dust mites, and certain foods.
Asthma: Similar to allergic reactions, asthma can lead to elevated eosinophils, and chronic inflammation may also increase monocyte counts.
Skin Conditions: Certain skin conditions like eczema (atopic dermatitis) can cause eosinophilia and potentially elevated monocytes due to chronic inflammation.

Hematological Malignancies: Although less common, certain blood cancers like leukemia (e.g., chronic myelomonocytic leukemia) can cause elevations in both monocytes and eosinophils. Missing this diagnosis could be fatal.
Lymphoma: Some types of lymphoma can present with eosinophilia and monocytosis, making it crucial not to overlook this possibility.
Hypereosinophilic Syndrome (HES): A rare condition characterized by overproduction of eosinophils, which can lead to organ damage. Elevated monocytes might be seen in the context of chronic inflammation or secondary infections.

Churg-Strauss Syndrome (Eosinophilic Granulomatosis with Polyangiitis): A rare autoimmune condition that can cause asthma, eosinophilia, and vasculitis, potentially affecting multiple organ systems.
Tropical Pulmonary Eosinophilia: A condition seen in some individuals infected with filarial parasites, leading to marked eosinophilia and pulmonary symptoms.
Idiopathic Eosinophilic Pneumonia: A rare condition characterized by eosinophilia and pulmonary infiltrates without a clear cause, which might also show elevated monocytes due to the inflammatory nature of the disease.