The number of humans infected with bird flu is on the rise.

Four new cases in Washington state this week bring the total number of human infections up to 31. Washington is the sixth state to report human cases of the disease, which has spread rapidly among poultry and cattle farms.

Experts say there’s a higher chance of a broad outbreak among humans if the virus remains uncontrolled in animals.

California, Colorado, Michigan, Missouri, and Texas have also reported human cases of bird flu.