Viral Meningitis: This condition is characterized by inflammation of the meninges (the protective membranes covering the brain and spinal cord) due to a viral infection. Symptoms such as headache, fatigue, nausea, and vomiting are common, and an elevated white blood cell (WBC) count can indicate an infectious process. Viral meningitis is often more prevalent and typically has a better prognosis than bacterial meningitis, making it a likely initial consideration.

Bacterial Meningitis: Although less common than viral meningitis, bacterial meningitis is a serious condition that presents with similar symptoms, including headache, nausea, vomiting, and an elevated WBC count. It requires prompt antibiotic treatment to prevent serious complications.
Urinary Tract Infection (UTI) or Pyelonephritis: Especially in patients with urinary symptoms, a UTI or pyelonephritis (infection of the kidney) could present with systemic symptoms like fatigue, nausea, and an elevated WBC count. The headache could be related to the systemic infection or fever.
Influenza or Other Viral Infections: Influenza and other viral infections can cause a range of symptoms including headache, fatigue, nausea, and vomiting, along with an elevated WBC count indicating the body's response to the infection.

Subarachnoid Hemorrhage: A life-threatening condition where there is bleeding into the space surrounding the brain. It can present with a sudden, severe headache (often described as "the worst headache of my life"), nausea, vomiting, and sometimes an elevated WBC count due to stress response. It requires immediate medical attention.
Brain Abscess: A rare but potentially life-threatening condition where an abscess (collection of pus) forms in the brain. Symptoms can include headache, nausea, vomiting, and changes in mental status, along with an elevated WBC count.
Encephalitis: Inflammation of the brain tissue, usually due to viral infections. Symptoms can include headache, fever, confusion, seizures, and sometimes nausea and vomiting. An elevated WBC count can be present, indicating an infectious process.

CNS Lymphoma: A type of cancer that affects the central nervous system. It can present with a variety of symptoms including headache, nausea, vomiting, and changes in mental status. While rare, it's an important consideration in patients with persistent or worsening symptoms.
Toxoplasmosis: An infection caused by the parasite Toxoplasma gondii. It can reactivate in immunocompromised individuals and cause neurological symptoms, including headache, confusion, and sometimes nausea and vomiting.
West Nile Virus: A viral infection transmitted by mosquitoes, which can cause meningitis or encephalitis. Symptoms include headache, fever, nausea, vomiting, and sometimes an elevated WBC count. It's more common in certain geographic areas during specific times of the year.