The virus has recently expanded in two ways: across regions and across species. Rather than being concentrated in Asia, bird flu has moved across much of the planet. And it has infected a wider variety of animals, including mammals. ( This Times story explains .) Dairy cows in many parts of the United States have tested positive.

H5N1 bird flu is spreading among animals and humans, raising concerns about mutation risks. While human cases are mild, experts stress urgent preparedness for potential pandemics and vaccine readiness gaps.

The developing avian influenza panzootic and the growing possibility of a pandemic bring to the fore the existential questions for humanity of climate change and the destruction of public health, both of which must be stopped in order to control the growing threats.

The recent appearance of the H5N1 bird flu in pigs does raise viral reassortment concerns. Here's what we know about this highly pathogenic avian influenza virus so far.

A recent study shows a mild increase in the H4N1 strain of bird flu transmissibility through air compared to older variants.