Viruses are mostly spread through the air, transmitting from wild birds into farms, due to lack of bio security; or by people and materials that have been involved in the eradication of the infected animals. Weather conditions also play an important role, like wind direction and temperature.
So once a farm is infected, the number of people and the materials needed for the eradication has to be limited to the minimum.
What goes wrong in many cases is that governments are not prepared because they didn’t want to invest in the correct techniques.
So when it happens, they are doomed to bring in as many people as they can, causing an enormous risk of reintroduction of the viruses.
Most of these people are poorly trained in bio security and using personal protection equipment correctly. If something goes wrong, they may become the next source of infection.