Following previous works, presumably 33 per cent of the infected people are asymptomatic (i.e. do not show any symptoms) while the remaining 67 per cent show symptoms of infection (Longini et al. 2005; Germann et al. 2006; Wu et al. 2006) . Of those who are symptomatic, 6 per cent are assumed critical and require hospitalization (Wu et al. 2006) . The other 94 per cent will show only mild symptoms. For the critically ill, they are 50 per cent more infectious as compared with those showing mild symptoms, while the asymptomatic cases are 50 per cent less infectious. In the current model, we ignored the occurrence of deaths. However, it can be assumed that the death rate is directly proportional to the total attack rate. For each individual, the susceptibility is assumed to be constant (unless the person is on medication, or immunized against the disease). However, the infectiousness (for a mildly symptomatic person) follows a baseline profile as shown in figure 1b. Both the susceptibility and infectiousness can be modified by the consumption of antiviral drugs. While an infected person is on medication, his/her infectiousness is reduced by 60 per cent. On the other hand, the susceptibility of an uninfected person will be reduced by 30 per cent. In addition, the probability of symptoms appearing will be reduced to a minimum of 23.45 per cent depending on the length of medication. Being on medication also shortens the duration of the disease by a maximum of 25 h if the consumption of drugs is sustained throughout the course of infection. Finally, once a person has recovered from the disease, he/she is assumed to be immune to further infection, and we set the susceptibility to 0 (see the electronic supplementary material for more details).