Epidemics can be related to seasonality of certain infectious. Seasonality may enter into any of the eight key elements of the system: (1) susceptible recruitment via reproduction, (2) transmission, (3) acquired immunity and recovery, (4) waning immunity, (5) natural mortality, (6) symptomatology and pathology (which may be acute or chronic, depending on the disease), (7) disease-induced mortality, and (8) cross-species transmission.  Influenza, the common cold, and other infections of the upper respiratory tract, such as sore throat, occur predominantly in the winter.