an accurate understanding of casualty rates until at least a year after it has peaked, although we know already that influenza deaths in children and young adults have been higher than in recent memory. Mathematical modelling had been usefully applied to this and other infectious disease problems. If a virus on average affects 2 people for every individual infected, its effective reproduction number  (R)  is 2 and it can be logically derived that at least 50% of the population must be immune to control an outbreak and lead to its extinction. The formula for the proportion needed to be immune is 1 minus (1 divided by R). The H1N1 2009 virus has an effective R closer to 1.5, so as an approximation we need at least one third of people to be immune for control. The proportion is however higher in certain crowded situations such as children in day care or schools as transmissibility is also higher in those situations. There are reports from countries suggesting that uptake of the new vaccine for H1N1 2009 may be somewhere between 10 -- 20% of the population. Almost all are likely to be immune as a consequence. If about 20% are also immune due to infection, then we may well not have enough population-wide protection to prevent the next wave, particularly in high risk settings like schools and kindergartens. The more I learn, the more I understand how much I don't know. What is more, as an epidemiologist, the biggest lesson I