Viral Meningitis: Given the symptoms of fever, malaise, headache, and recent onset of seizure-like activity, viral meningitis is a strong consideration. It is relatively common in adolescents and can present with these symptoms, especially if the headache has worsened over time and is accompanied by fever and neurological symptoms.

Bacterial Meningitis: Although less common than viral meningitis, bacterial meningitis is a critical diagnosis to consider due to its high morbidity and mortality if not treated promptly. Symptoms can include fever, headache, malaise, and neurological deficits, including seizure-like activity.
Encephalitis: Inflammation of the brain tissue, often due to viral infections, can present with fever, headache, malaise, and neurological symptoms such as seizures. It's a plausible diagnosis given the seizure-like activity and the duration of symptoms.
Intracranial Abscess: A collection of pus in the brain, usually as a result of infection, can cause fever, headache, and neurological symptoms, including seizures. It's less common but should be considered, especially if there's a history of recent infection or trauma.

Subarachnoid Hemorrhage: Although less likely, a subarachnoid hemorrhage (bleeding into the space surrounding the brain) can present with sudden severe headache and can lead to seizure-like activity. It's critical to diagnose promptly due to its high mortality rate.
Brain Tumor: A tumor in the brain can cause increased intracranial pressure, leading to symptoms such as headache, malaise, and seizures. While less common, it's a diagnosis that cannot be missed due to its significant implications for treatment and prognosis.
Cerebral Vasculitis: Inflammation of the blood vessels in the brain can lead to a variety of symptoms, including headache, fever, and neurological deficits. It's a rare condition but one that requires prompt diagnosis and treatment to prevent serious complications.

Neurocysticercosis: An infection caused by the pork tapeworm that can lead to cysts in the brain, causing seizures, headache, and other neurological symptoms. It's more common in areas where pork tapeworm infection is prevalent.
Rasmussen's Encephalitis: A rare inflammatory neurological disease, characterized by frequent and severe seizures, loss of motor skills, and speech, typically affecting one side of the brain. It's a rare condition but should be considered in cases of unexplained seizures and neurological deterioration.
MELAS Syndrome: A rare mitochondrial disorder that can cause seizures, headache, and other neurological symptoms due to mitochondrial dysfunction. It's a diagnosis to consider in cases where there's a suspicion of a mitochondrial disorder.