The agricultural origins of pandemics

I’m slowly getting my head around how the arrangement of agriculture, with its co-ordination of animals in time and space, creates the conditions in which viruses are incubated, as Catharine Arnold explains in her Pandemic 1918:

Little did doctors suspect, during the First World War, that ducks operated as a ‘reservoir’ for bird flu viruses, littering the soil with faeces that were then snorted in by pigs grubbing for food, and that the pigs would subsequently incubate avian viruses and combine them with the human flu viruses acquired through contact with people. It would not be until the last decades of the twentieth century that virologists such as Professor John Oxford and Jeffery Taubenberger discovered that avian flu could leap the species barrier and mutate into an influenza virus capable of infecting and killing humans.

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