To better answer your question, there would need to be more context. Also, please know that this is not the best way to obtain a diagnosis. That is best left to the health care providers tending to the case.
With that in mind, I would be inclined to test for viruses (and bacteria) that cause meningitis. The stiff neck, fever, and neural involvement are classic of meningitis. Viruses that cause meningitis are may, but the most common ones are herpes viruses. Bacteria that cause meningitis are also many, but the most common are Neisseria meningitidis, Pneumococcal meningitis, and Haemophilus influenzae type B.