Viral Gastroenteritis: This is often the most common cause of headache and vomiting, especially if the symptoms are accompanied by fever, diarrhea, or abdominal cramps. The headache is usually a result of dehydration and the body's response to the infection.

Migraine: Migraines can cause severe headaches and are often accompanied by nausea and vomiting. They can be triggered by various factors, including stress, certain foods, or hormonal changes.
Tension Headache: While typically less severe than migraines, tension headaches can also lead to nausea and vomiting, especially if the pain is intense and persistent.
Gastroesophageal Reflux Disease (GERD): GERD can cause nausea and vomiting, and the discomfort or pain from acid reflux can lead to headaches.

Meningitis: Although less common, meningitis is a critical diagnosis that must not be missed. It presents with severe headache, fever, and vomiting, and can progress rapidly to seizures, coma, and death if not treated promptly.
Subarachnoid Hemorrhage: A sudden, severe headache (often described as "the worst headache of my life") accompanied by vomiting can be a sign of a subarachnoid hemorrhage, which is a life-threatening condition requiring immediate medical attention.
Increased Intracranial Pressure: Conditions that increase pressure inside the skull, such as a brain tumor or hydrocephalus, can cause headaches and vomiting. These conditions are medical emergencies.

Brain Abscess: A rare but serious condition where an abscess forms in the brain, causing headache, vomiting, and potentially severe neurological symptoms.
Encephalitis: Inflammation of the brain tissue, usually due to a viral infection, which can cause headache, vomiting, confusion, and seizures.
Pseudotumor Cerebri (Idiopathic Intracranial Hypertension): A condition characterized by increased pressure within the skull without any apparent cause, leading to headaches and vomiting, particularly in young, obese women.