The bird virus, designated D1.1, has clearly gone through reassortment, a mixing of different viruses, presumably in a bird infected with two different avian influenza viruses.

Through a process of virus mutation called reassortment, that flu bug might pass on an aptitude for human-to-human transmission to the H5N1 virus.

This is of particular concern amid the standard influenza season because genes could swap and mutate in an organism infected with both the seasonal flu and bird flu in a process called viral reassortment.

The virus was also detected in pigs for the first time, which is particularly concerning because pigs are known as “mixing vessels,” as they can contract both human and avian pathogens, increasing the chances of viral reassortment.

In the 2009 swine flu pandemic, multiple reassortment events in pigs and birds led to the novel H1N1 virus strain, which led to 60 million cases and 12,000 deaths in the U.S. in its first year of circulating, per the CDC.