Meningitis: This is the most likely diagnosis given the combination of fever, confusion, and neck stiffness. Meningitis is an inflammation of the protective membranes covering the brain and spinal cord, known as the meninges, and these symptoms are classic presentations.

Encephalitis: An inflammation of the brain tissue, often caused by viral infections. Symptoms can include fever, confusion, and sometimes neck stiffness, making it a plausible diagnosis.
Subarachnoid Hemorrhage: Although more commonly associated with a sudden, severe headache, confusion and neck stiffness can occur, especially if there is blood in the subarachnoid space irritating the meninges.
Brain Abscess: A collection of pus, immune cells, and other material in the brain, usually from a bacterial or fungal infection. Symptoms can include fever, confusion, and sometimes signs of increased intracranial pressure, which might mimic neck stiffness.

Septicemia: Bacteria in the blood can lead to a systemic infection that causes fever and confusion. While neck stiffness might not be a primary symptom, the severity of septicemia necessitates its consideration to prevent missing a potentially fatal condition.
Status Epilepticus: Prolonged or recurrent seizures without full recovery between them. While primarily presenting with seizure activity, post-ictal states can cause confusion, and in rare cases, fever and neck stiffness might be observed, especially if there's an underlying CNS infection.
Cerebral Vasculitis: Inflammation of the blood vessels in the brain, which can present with a wide range of symptoms including fever, confusion, and sometimes signs that might be mistaken for neck stiffness due to the involvement of the central nervous system.

Neurosyphilis: A complication of untreated syphilis infection that can occur years after the initial infection. It can cause a wide range of neurological symptoms, including fever, confusion, and sometimes neck stiffness, although these are less common presentations.
Lymphocytic Choriomeningitis (LCM): A viral infection that can cause meningitis or encephalitis, leading to symptoms such as fever, confusion, and neck stiffness. It's rare and usually associated with exposure to rodents or their droppings.
West Nile Virus: A mosquito-borne virus that can cause encephalitis, leading to fever, confusion, and sometimes neck stiffness. While more commonly associated with milder symptoms, severe neurological manifestations can occur, especially in older adults or those with compromised immune systems.