Cows are getting a new disease, bird flu, which started in wild birds but is now moving to cows and possibly people.
Scientists are worried because the bird flu virus is changing and could spread more easily to humans.
To help stop the spread, hundreds of thousands of dairy cows will need to be tested before they can move to other farms.
People should avoid drinking raw milk and dairy farm workers should take extra precautions like wearing masks and washing their hands.
