Millions of chickens and turkeys have been killed in the Midwest this week in a desperate attempt to stop the spread of bird flu.

On Friday afternoon, Iowa Governor Terry Branstad declared a state of emergency after four new poultry farms tested positive for the virus.

The outbreak, which appears to be the deadliest for birds in United States history, has spread to 13 states and across the border into Canada. Humans do not seem to be at risk from this outbreak, but any avian flu has the potential to mutate into a disease as dangerous to people as it is to poultry.

National Geographic asked experts about the current outbreak and how much risk it poses to humans:

The influenza virus