The quantity that measures the epidemic potential of a pathogen is the basic reproduction number R 0 [11, 12] . R 0 is defined as the average number of new infections created by an infectious individual in an entirely susceptible population. For established human pathogens, leading to standard epidemics, R 0 .1, as is the case of seasonal or pandemic influenza [13] [14] [15] [16] [17] [18] [19] . In practice, epidemiological data typically permit only the estimation of the effective reproduction number R, which may differ from R 0 due to acquired immunity and other factors. For an emerging infectious disease, when transmission is only incipient [20] and the pathogen is adapting to the population, it becomes crucial to monitor quantitative changes of the effective reproduction number over time. Thus, the detection and tracking of an emerging disease can be formalized in terms of monitoring R, as it evolves and approaches the critical threshold RR1. This is likely the current state of H5N1 avian influenza in humans, where complete absence of human to human transmission would imply R = 0, but likely R is very small, as a few cases of possible human contagion suggest [21] [22] [23] .