The 2015 avian influenza epidemic was the largest in U.S. history, affecting more than 48 million domestic poultry birds in 15 states between December 2014 and June 2015, according to the U.S. Department of Agriculture's Animal and Plant Health Inspection Service.

When considering the 2015 avian influenza epidemic in the U.S., it's important not to assume that the disease spread according to any one neat geographical pathway.

This year's avian influenza epidemic put Wisconsin agriculture officials through one of the toughest tests they've faced in a long time.