• Subarachnoid hemorrhage
• Encephalitis
• "Neighborhood reaction" causing abnormal CSF, eg, brain abscess, epidural abscess, vertebral osteomyelitis, mastoiditis, sinusitis, brain tumor
• Dural sinus thrombosis
• Noninfectious meningeal irritation: carcinomatous meningitis, sarcoidosis, systemic lupus erythematosus (SLE), drugs (eg, NSAIDs, TMP-SMX), pneumonia, shigellosis
• If fever and rash: gonococcemia, infective endocarditis, thrombotic thrombocytopenic purpura (TTP), Rocky Mountain spotted fever, viral exanthem
• Seizure due to other causes, eg, febrile seizure
• Amebic meningoencephalitis (Naegleria fowleri)

• Viral encephalitis
• Neck stiffness
• Seizure
• Fever and rash
• Altered mental status
• Headache and fever
• Headache and nausea
• Neck pain and fever
• Nausea and fever
• Altered mental status and fever
• Headache and fever

Purulent meningitis

• 18-50 years: S. pneumoniae, N. meningitidis
• >50 years: S. pneumoniae, N. meningitidis, Listeria, gram-negative bacilli
• Impaired cellular immunity: Listeria, gram-negative bacilli, S. pneumoniae
• Postsurgical or posttraumatic: S. aureus, S. pneumoniae, gram-negative bacilli

Aseptic meningitis

• Mumps, coxsackievirus, echoviruses
• Infectious mononucleosis
• Leptospirosis, syphilis, Lyme disease

Chronic meningitis

• TB or atypical mycobacteria
• Fungi: Cryptococcus, Coccidioides, Histoplasma
• Spirochetes: syphilis, Lyme disease, leptospirosis
• Other: brucellosis, HIV infection, neurocysticercosis