What is the H5N1 bird flu? Will it cause the next big pandemic? If so, how bad would that pandemic be?

Flu is a disease caused by a family of related influenza viruses. Pandemic flu is always caused by the influenza A virus. Influenza A has two surface antigen proteins, hemagglutinin (18 flavors) and neuraminidase (11 flavors). A particular flu strain is named after which flavors of these two proteins it has - for example, H3N2, or H5N1.

Influenza A evolved in birds, and stayed there for at least thousands of years. It crossed to humans later, maybe during historic times - different sources give suggest dates as early as 500 BC or as late as 1500 AD. It probably crossed over multiple times. Maybe it died out in humans after some crossovers, stuck around in birds, and crossed over from birds to humans again later.

https://www.astralcodexten.com/p/h5n1-much-more-than-you-wanted-to