[Possible causes] Meningitis, encephalitis (inflammation of the brain tissue), Lyme disease, collagen vascular disease A stiff neck, or inability to flex the neck due to pain, indicates inflammation of the meninges. Other signs of systemic illness indicates infection. [Diagnostic tests] Neuroimaging, lumbar puncture, serology (diagnostic blood tests for infections) [Red Flag] 
Headache with signs of total body illness (fever, stiff neck, rash)