"In all of God's sweet aviary there exists no bird more diabolical and ruthless than the egg-laying chicken hen. Despite the darkness of this clucking beast's heart, our nation's press has gone on a rampage insisting that more and more citizens everywhere in the United States are choosing to board and feed these creatures in their urban and suburban backyards so they can harvest the eggs."
I don't know for sure, but would keep my flock healthy the same way you keep yourself healthy with good diet, lacto-fermented foods, garlic, herbs, low stress, etc.
This is recommended for people and you can try it with the chickens:
If you hear there is flu around the area you can try supplementing their diet with Echinacea, Astragalus, Garlic, Elder flower and berry, St. John's Wort (in a study in Vietnam it killed 99& of H5N1 virus), Lemon Balm, Spilanthes, Ginger
If there is flu in your town- Curcumin (in Turmeric), resveratrol (in grapes and the root of the invasive Japanese Knotweed- make medicine from invasives! They are coming around for a reason) green tea, boneset