Bird flu, also known as avian influenza or H5N1, has infected more than 130 million birds over the past two years as it threatened commercial bird operations and recently led to the first human death in the United States.

How deadly is the bird flu? The US has reported its first death from avian influenza, H5N1. Here's what to know about bird flu mortality rate and who is at risk.

The patient, who was older than 65 and had underlying medical conditions, was hospitalized weeks ago in critical condition with severe respiratory illness.