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Pandemic Flu | NOVA | PBS

https://www.pbs.org/wgbh/nova/sciencenow/3302/04.html

In a bird flu, the bird virus has an instruction that says "Infect the gut, and give the bird diarrhea, so it leaves droppings.". And mostly it's the droppings that transmit the virus for birds. So we do it here, they do it there. The bird virus, therefore does not need an instruction for coughing and sneezing.