We conclude that the variable application of social distancing, whereby individuals reduced their infectious contact rate in response to the perceived risk, is a plausible explanation for the multiple waves of pandemic strain influenza seen during 1919 in Sydney, Australia. Indeed, while the waxing and waning of the multiple waves appears dramatic, the degree of social distancing required to explain this (in this case, at most, halving one's infectious contact rate) seems quite possible. More generally, Bootsma & Ferguson (2007) and Hatchett et al. (2007) have demonstrated that variation in the timing of introduction and lifting of non-pharmaceutical interventions aimed at reducing contact rates can explain why cities experienced different inter-wave periods, ranging from being so short as to be undetectable through to several months (Taubenberger & Morens 2006) . We note, however, that transient social distancing certainly does not explain why the case-fatality rate of the 1918-1919 pandemic typically was higher during the second wave, as indeed was the case for Sydney (McCracken & Curson 2003) . However, note that the very similar reproduction numbers observed during both waves of the epidemic support our initial assumption that R 0 did not differ over the course of the epidemic. Subject to the assumption that infection at any time conferred protection against a subsequent severe attack, we conclude that approximately 9% of the population were resistant to the epidemic strain prior to the epidemic, and that, during the epidemic, not more than approximately 9% of infections that conferred resistance to the epidemic strain were subclinical to the extent that people were able to continue working. Using our best estimate that 91.2% of individuals were initially susceptible, the R 0 of the 1919 influenza epidemic in Sydney was 1.8, consistent with recent estimates that have used a similar mean serial interval (Ferguson et al. 2005; Sertsou et al. 2006) . The observed attack rate, however, was substantially less than would be expected for this basic reproduction number, and we argue that social distancing is a plausible reason for this. This result underlines the effective role that social distancing could possibly play in mitigating the effects of a future pandemic of influenza. Novel Inhibitor Design for Hemagglutinin against H1N1 Influenza Virus by Core Hopping Method The worldwide spread of H1N1 avian influenza and the increasing reports about its resistance to the current drugs have made a high priority for developing new anti-influenza drugs. Owing to its unique function in assisting viruses to bind the cellular surface, a key step for them to subsequently penetrate into the infected cell, hemagglutinin (HA) has become one of the main targets for drug design against influenza virus. To develop potent HA inhibitors, the ZINC fragment database was searched for finding the optimal compound with the core hopping technique. As a result, the Neo6 compound was obtained. It has been shown through the subsequent molecular docking studies and molecular dynamic simulations that Neo6 not only assumes more favorable conformation at the binding pocket of HA but also has stronger binding interaction with its receptor. Accordingly, Neo6 may become a promising candidate for developing new and more powerful drugs for treating influenza. Or at the very least, the findings reported here may provide useful insights to stimulate new strategy in this area. In recent years, severe flu-like human cases were reported around the world and subsequently the causative virus was identified as the influenza A virus [1, 2] . The virus was spreading rapidly around the world and had been identified as a new reassortant with three genetic lineages, mainly with a swine origin. Therefore, it was called swine-origin influenza virus (S-OIV). Owing to its extremely rapid human-to-human transmission rate, within only two months the 2009 S-OIV had been detected throughout the entire world. On June 11th, 2009 the World Health Organization (WHO) declared an official pandemic, the first pandemic in the 21st century [3] .