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the pathogen to survive outside the host - e.g. water-borne cholera which becomes prevalent in tropical wet seasons, or influenza which peaks in temperate regions during winter.
* The behaviour of people susceptible to the disease - such as spending more time in close contact indoors.
* Changes in immune function during winter - one possibility is a reduction in vitamin D, and another is the effect of cold on mucous membranes in the nose.
* Abundance

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