Also known as avian influenza, bird flu is an infection from a type of virus that spreads in birds, animals, and sometimes even humans and is much more likely to be deadly. However, it is extremely rare for it to spread from person to person. There are many subtypes of avian flu and the most common ones that spread to humans are influenza A (H5N1) and influenza A (H7N9). These are named based on the types of proteins on the surface of the virus.

In the US till now, only one human case of bird flu has been confirmed - a prisoner in a work program picked it up while killing infected birds at a poultry farm in Montrose County, Colorado. His only symptom was fatigue, and he recovered.