I say flies but I mean insects in general. Are they subject to the same ills as the rest of us? Can they get whooping cough, mumps, flu, coughs, all the annoying things that plague us humans? Or do they have their own microbes to worry about?
The flu can affect other animals. The “bird flu” is called the bird flu because that virus exists in large numbers in birds. “Swine flu” is present in pigs. The influenza virus has been found in dogs, cats, seals, bats, and even whales.
Insects don’t get the flu, but they do have their own diseases to worry about:
https://www.scientificamerican.com/article/what-kind-of-illnesses-do/
Much more info in the linked article.
Fleas are sickened by plague bacilli, which they regurgitate while feeding on humans (not a good thing).
Not precisely an insect, but guinea pigs are known to sneeze.
There are plenty of bacterial, viral, and fungal diseases of insects. Pretty much all plants and animals are infected by some kind of microbe.
Of course insects can’t get the kind of symptoms listed in the OP, such as coughs, since they have a completely different kind of respiratory system than vertebrates. They can’t get a runny nose either because they don’t have noses. And influenza itself is confined to birds and mammals.
Thank you, guys, great links! That Scientific American one deals with something that really puzzled me, ie how do we know they’re sick unless it radically affects behaviour?