A patient presents with fever, headache, and URI symptoms. What is the most likely diagnosis?

The presentation of fever, headache, and upper respiratory tract infection (URI) symptoms aligns closely with influenza. Influenza commonly manifests with systemic symptoms such as fever and headache, along with respiratory symptoms like a cough, sore throat, and nasal congestion. This condition is characterized by its abrupt onset and will often peak rapidly, which fits the scenario described.

While COVID-19 can also present with similar symptoms, its range of symptoms and sometimes milder presentation can make it less definitive compared to influenza during certain seasons when influenza is more prevalent. Additionally, while pertussis may cause a cough and respiratory symptoms, it typically has a catarrhal phase followed by paroxysmal coughing, which does not match the initial presentation of straightforward URI symptoms. Atypical mycobacterial disease, while causing non-specific symptoms, is less common in this context and would not typically present with such acute URI symptoms. Thus, influenza is the most likely diagnosis based on the symptoms described.