New strain found among people who recently attended fairs.
A new strain of swine flu has emerged, with 29 cases in humans diagnosed since last summer.
Fair organizers are taking extra precautions now that the new strain is spreading from pigs to people, and has started picking up speed.
Recent human cases have been found in Indiana, Ohio and Hawaii, mostly after direct contact with sick pigs at county fairs.
This is different from the H1N1 swine flu that circulated in 2009.
This strain is called H3N2.
The most notable difference, other than its name, is that it does not appear to spread very well from person to person.
"We're always concerned about the potential for that virus to spread efficiently between humans. We're not seeing that right now, but we'll keep monitoring closely," says Dr. Joseph Bresee of the Centers for Disease Control.
The new flu has hit children, mostly, and people with underlying health problems.
The illness is treatable with medications if needed, and most patients have not been hospitalized.
This year's flu shot does not protect against H3N2.
Experts say the best way to avoid it is to wash your hands before and after contact with animals.