Viral infection: This is the most likely diagnosis, as many viral infections, such as influenza or upper respiratory tract infections, can present with high fever as the initial or sole symptom.

Bacterial infection: Bacterial infections like pneumonia or urinary tract infections can also cause high fever, although they often present with additional symptoms.
Influenza: Influenza is a common cause of high fever, especially during flu season.
Mononucleosis: Infectious mononucleosis, caused by Epstein-Barr virus, can present with fever as the primary symptom, especially in the early stages.

Meningitis: Although less likely, meningitis is a potentially life-threatening condition that can present with fever as the initial symptom, making it crucial not to miss.
Sepsis: Sepsis, a severe response to infection, can initially present with fever and may not have other obvious symptoms early on.
Endocarditis: Infective endocarditis, an infection of the heart valves, can present subtly with fever and may not have other symptoms initially.
Tuberculosis: TB can cause prolonged fever with minimal other symptoms, especially in its early stages or in immunocompromised individuals.

Lymphoma: Certain types of lymphoma can present with prolonged fever as the primary symptom (fever of unknown origin).
Factitious fever: This is a rare condition where a person intentionally produces or falsifies fever, often for attention or to avoid work.
Autoimmune disorders: Some autoimmune disorders, like adult-onset Still's disease or systemic lupus erythematosus, can present with recurrent or persistent fever as a primary symptom.
Familial Mediterranean fever: A genetic disorder that causes recurrent episodes of fever, often accompanied by other symptoms like abdominal or chest pain, but can sometimes present with fever alone.