Viral Infection (e.g., Influenza): This is often the most common cause of high fever, headache, arthralgia, and leukocytosis, especially during flu season or in the context of a viral outbreak. The symptoms presented are classic for a viral infection, which typically induces a robust immune response leading to fever and leukocytosis.

Bacterial Infections (e.g., Pneumonia, Urinary Tract Infections): Bacterial infections can cause similar symptoms, including high fever, headache, and arthralgia, especially if the infection is systemic or severe. Leukocytosis is a common finding in bacterial infections due to the body's immune response.
Autoimmune Disorders (e.g., Rheumatoid Arthritis, Lupus): While these conditions may not always present with fever, they can cause arthralgia and systemic symptoms. Leukocytosis can occur, especially during active disease phases.
Malaria: In endemic areas, malaria should be considered, especially if the patient has recently traveled. It presents with cyclical fevers, headache, and can cause arthralgia and leukocytosis.

Meningitis: Although less common, meningitis is a medical emergency that can present with fever, headache, and sometimes arthralgia. Leukocytosis is expected, and prompt diagnosis and treatment are crucial to prevent serious complications or death.
Sepsis: Sepsis, particularly from bacterial infections, is a life-threatening condition that can present with fever, headache, and systemic symptoms including arthralgia. Leukocytosis is a key finding, and early recognition and treatment are vital.
Endocarditis: Infective endocarditis can cause fever, systemic symptoms, and sometimes arthralgia. It's a serious condition that requires prompt diagnosis and treatment to prevent severe complications.

Lyme Disease: In endemic areas, Lyme disease can cause fever, headache, arthralgia, and leukocytosis, especially in its early stages. It's less common than viral or bacterial infections but should be considered in patients with exposure history.
Brucellosis: This zoonotic infection can cause fever, headache, arthralgia, and leukocytosis. It's rare in many parts of the world but should be considered in individuals with occupational exposure to animals or travel to endemic areas.
Ehrlichiosis/Anaplasmosis: These tick-borne illnesses can present with fever, headache, and arthralgia, along with leukocytosis. They are less common but can be severe and should be considered in patients with tick exposure.