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Lecture 9

BIO220 Lecture 9.docx

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Doug Thomson

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BIO220 Lecture 9 – February 7, 2012 The Evolution of Virulence II Stage 1: Accidental Infection  Pathogens cause infection in new, unfamiliar hosts o Infections may die out quickly because it is so virulent it kills its host quickly  Accidental infections can be quite virulent o Low virulent infections aren’t going to be reported...could be that highly virulent infections are overestimated with respect to low virulent species Stage 2: Virulence soon after invasion  If a virus is poorly adaptedstrong selection by the hostrapid evolution Stage 3: Evolution of optimal virulence  High transmission rate but low transmission period trade off  Total transmission of the infection is expected to be maximized Evolution of Influenza Hits at winter every year, here and in Australia  Why? Because in winter, we are all in close proximity and transmission more easily done The death toll  Hard to get solid numbers because individuals who get the flu don’t die from the flu but rather from secondary effects  We all eventually get immunity from the strain of flu that we have once had The flu is evolving very fast so every season there is a slightly different flu strain  Recent flu strains are descendent from a single ancestor –every year it is changing but it is still the direct descendent Drift – the genotype of the virus is drifting when the errors of the virus through evolution accumulate  Hypothesis - the one that has the most new mutations should be able to evade the immune system the best  Prediction – you can look at this year’s strain and predict next year’s strain  Results – the human immune response is playing a stro
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