(1) Exposed individuals are infective during the incubation period. It is commonly known (e.g., [18] ) that the pre-symptomatic (exposed) individuals cannot transmit Hsieh BMC Infectious Diseases 2010, 10:106 http://www.biomedcentral.com/1471-2334/10/106 the disease in the noninfectious latent period, during which the viral titres gradually increase to detectable and transmissible levels when they became infective for only a short period (0.25 days in [19] ) before the onset of symptoms. To avoid adding an extra compartment to account for the (short) period of infectivity after the end of the latent period and before the end of incubation period (see e.g., [20] ), we assume that the individuals are infective during the incubation period with the infectivity averaged out over the whole incubation period. This simplification is reasonable since the incubation period for influenza is typically very short, e.g., 1.48 days in [21] .