Within a country, the disease spreads easily from farm to farm. Large amounts of virus are secreted in bird droppings, contaminating dust and soil. Airborne virus can spread the disease from bird to bird, causing infection when the virus is inhaled. Contaminated equipment, vehicles, feed, cages or clothing - especially shoes - can carry the virus from farm to farm. The virus can also be carried on the feet and bodies of animals, such as rodents, which act as "mechanical vectors" for spreading the disease. Limited evidence suggests that flies can also act as mechanical vectors.

Droppings from infected wild birds can introduce the virus into both commercial and backyard poultry flocks. The risk that infection will be transmitted from wild birds to domestic poultry is greatest where domestic birds roam freely, share a water supply with wild birds, or use a water supply that might become contaminated by droppings from infected wild-bird carriers.

So called "wet" markets, where live birds are sold under crowded and sometimes unsanitary conditions, can be another source of spread.

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