While flu cases still occur throughout the year, influenza epidemics usually occur during the colder months in temperate regions. Multiple factors may contribute to this, including:
People spending more time indoors, creating more opportunities for the virus to spread.
Lack of sunlight, which results in reduced vitamin D production leading to compromised immune function.
The influenza virus itself may survive better in colder, drier conditions.
Clearance of mucous (and the pathogens trapped within) from the airways may be impaired by cold conditions.