In January, two school districts saw a sudden increase in absences. At the same time, sales of over-the-counter medications for fever, cough, and cold symptoms increased dramatically. To determine the etiology of this outbreak of respiratory illness, the public health department conducted a survey of local physicians to see what types of patients they were currently seeing most. The doctors all reported increased numbers of patients complaining of abrupt onset of high fever, severe headache, and myalgia followed by sore throat, dry cough, weakness, and severe fatigue. The patients were ill for 3 to 5 days, but many reported persistent malaise. What is the most likely diagnosis for this outbreak?