Most seasonal influenza outbreaks in the northern hemisphere occur between December and February, according to the Centers for Disease Control and Prevention. However, there may be cases that occur as early as October and as late as May.
The peak month for influenza is the month that has the highest percentage of respiratory specimens that test positive for the flu virus, reports the CDC. During a 32-year period between 1982 and 2014, flu activity peaked 14 times in February, six times in December, five seasons in January and five seasons in March. The CDC uses information provided by state and local health departments, physicians offices, clinical laboratories, and emergency rooms to compile a weekly activity report of influenza in the United States.