Other limitations are: (1) While before the start of the novel A/ H1N1 epidemic it was difficult to make realistic assumptions, it is now easier to do so now, since first estimates can be drawn for a number of parameters of the novel influenza virus. Nevertheless, many pieces of information are uncertain and may change due to more information coming to light, or due to real changes of the virus and its epidemiology. (2) The effect of season is not taken into account, we expect that the virus is more easily transmitted in the fall or winter time [18, 19] . (3) The proportion of asymptomatic cases and their contribution to transmission is still unknown. (4) Lacking realistic alternative information we distinguished only two age groups, children and adults. (5) The evolution of the number of cases imported is unknown and will probably change over time. (6) The age dependence of susceptibility in Germany is unknown and is likely to differ from the one in Mexico. (7) The sensitivity of surveillance is unknown, and therefore the true proportion of cases detected is also unknown. If persons were infected in Germany from a source without travel history, then they may have been more easily missed, especially since initial surveillance efforts usually focus on diseased persons with travel history. (8) The rigor of public health measures is likely to vary among different local health departments. However, it is plausible that the measures taken contributed to the delay of the initial spread of the infection, because, until mid June at latest, virtually no tertiary cases had been detected by close surveillance of contacts and the surrounding of cases.