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Wisconsin Governor Scott Walker declared a state of emergency Monday because of an outbreak of H5N2 bird flu that kills poultry, activating the state's National Guard to help fight the infection.

Wisconsin is one of about a dozen U.S. states to be hit by the highly pathogenic strain of H5N2, which experts believe is spread by migrating waterfowl. Canada has been affected, also.

The virus has been found in three Wisconsin poultry flocks, affecting tens of thousands of chickens and turkeys. It can sweep through commercial flocks, wiping them out in days.

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“We must act quickly and efficiently to contain the outbreak and protect domestic poultry,” Walker said in a statement. “It is important to note, however, there is no threat to humans with the avian flu outbreak.”

"There is no threat to humans with the avian flu outbreak.”

There are dozens of strains of avian influenza, many of which don’t cause severe symptoms in birds. But highly pathogenic strains can wipe out flocks of valuable commercial poultry and spread quickly, so outbreaks mean flocks must be destroyed.

They also very occasionally can spread to people. H5N1 bird flu is the most feared strain. The World Health Organization says more than 780 people in 16 countries are known to have been infected with H5N1 since 2003 and more than 400 of them have died. A second strain called H7N9 has infected 622 people since 2013 and killed 227 of them.

Since H5N1 started spreading in 2003, health and agricultural officials have been on high alert for any type of highly pathogenic avian influenza. In the past few months, H5N2 has hit 12 U.S. states including flocks in Arkansas, Washington, Oregon, Idaho, North Dakota, South Dakota, Minnesota, and a wild goose in Wyoming.

A different strain called H5N8 was found in a commercial flock of turkeys in California. H5N8 has also affected flocks in South Korea, Germany, Italy, Britain and the Netherlands.

The H5N2 and H5N8 strains are not considered a threat to people. But people can and do catch flu from animals. A strain of H3N2 influenza that infected some people has been traced to pigs at state fairs and most cases of both H7N9 and H5N1 bird flu are traced to people who have had direct contact with poultry.