Viral Upper Respiratory Infection (URTI): This is the most common cause of headache and fever in a young adult, often presenting with additional symptoms such as sore throat, runny nose, and cough, but these can be minimal or absent in early stages.

Sinusitis: An infection of the sinuses can cause headache and fever, often accompanied by facial pain and purulent nasal discharge.
Influenza: Especially during flu season, influenza can present with headache, fever, and may initially have minimal other symptoms.
Meningitis: Although less common, it's a significant consideration, especially if the headache is severe and accompanied by neck stiffness or photophobia.

Meningitis: As mentioned, this is critical to identify early due to its high morbidity and mortality if not promptly treated.
Encephalitis: An inflammation of the brain, usually caused by viral infections, which can present with headache, fever, and altered mental status.
Subarachnoid Hemorrhage: A life-threatening condition that can cause a sudden, severe headache (often described as "the worst headache of my life") and may be accompanied by fever if there's associated infection or inflammation.

Brain Abscess: A rare but serious condition where an abscess forms in the brain, usually as a result of a bacterial or fungal infection, which can cause headache, fever, and neurological deficits.
Temporal Arteritis: Although more common in older adults, it can occur in younger individuals and presents with headache, fever, and jaw claudication.
Lymphoma or Leukemia: In rare cases, systemic cancers can present with non-specific symptoms such as headache and fever due to CNS involvement or paraneoplastic syndromes.