Migraine headache
Tension headache
Cluster headache<unk> /encephalitis
Neoplastic<unk> 
Myocardial ischemia
Retropharyngeal abscess
Intracranial Hemorrhage (ICH)
SAH / sentinel bleed
Acute obstructive hydrocephalus
Space occupying lesions
CVA
Carbon monoxide poisoning
Basilar artery dissection
Preeclampsia
Cerebral venous thrombosis
Hypertensive emergency
Depression
Neoplastic<unk> 
SAH / sentinel bleed
Giant cell arteritis of temporal artery (temporal arteritis)
Idiopathic intracranial hypertension (Pseudotumor Cerebri)
Acute Glaucoma
Acute sinusitis
Cavernous sinus thrombosis or cerebral sinus thrombosis
Carotid artery dissection
Trigeminal neuralgia
TMJ pain
Post-lumbar puncture headache
Dehydration
Analgesia abuse
Various ocular and dental problems
Herpes zoster ophthalmicus
Herpes zoster oticus
Cryptococcosis
Febrile headache (e.g. pyelonephritis, nonspecific viral infection)
Ophthalmoplegic migraine
Superior Vena Cava Syndrome
Viral
Varicella
Herpes
Enterovirus
West Nile
Tuberculosis
Lyme disease
Syphilis
Leptospirosis
Fungal (AIDS, transplant, chemotherapy, chronic steroid use)
Noninfectious
Sarcoidosis
Vasculitis
Connective tissue disease
Drug induced aseptic<unk> 
Varicella
Herpes
Enterovirus
West Nile
Sarcoidosis
Vasculitis
Connective tissue disease
Drug induced aseptic<unk> 
Infectious
Sepsis<unk> 
Encephalitis
Cerebral malaria
Brain abscess
Other
Neuroleptic malignant syndrome
Serotonin syndrome
Malignant hyperthermia
Sympathomimetic toxicity (cocaine, amphetamine, ketamine)
Anticholinergic toxicity
Heat stroke
Delirium tremens
Hypothalamic stroke
Pheochromocytoma
Thyroid storm
Sepsis<unk> 
Encephalitis
Cerebral malaria
Brain abscess
Neuroleptic malignant syndrome
Serotonin syndrome
Malignant hyperthermia
Sympathomimetic toxicity (cocaine, amphetamine, ketamine)
Anticholinergic toxicity
Heat stroke
Delirium tremens
Hypothalamic stroke
Pheochromocytoma
Thyroid storm
Meningitis
Migraine headache
Encephalitis
Tension headache
Meningoencephalitis
Cluster headache
Neoplastic meningitis
Meningitis/encephalitis