ATLANTA — Those cute mail-order chicks that wind up in children's Easter baskets and backyard farms have been linked to more than 300 cases of salmonella in the U.S. — mostly in youngsters — since 2004.

An estimated 50 million live poultry are sold through the mail each year in the United States in a business that has been booming because of the growing popularity of backyard chicken farming as a hobby among people who like the idea of raising their own food. But health officials are warning of a bacterial threat on the birds' feet, feathers, beaks and eggs.

"Most people can tell you that chicken meat may have salmonella on it," said Casey Barton Behravesh of the Centers for Disease Control and Prevention. "But surprisingly, we found many people are not aware that live chicks and chickens can spread salmonella to people."

Since 2004, at least 316 people in 43 states got sick in an outbreak tied primarily to one mail-order hatchery. Health officials believe thousands more illnesses connected to the business were probably never reported.

No one died, but three dozen people were hospitalized with bloody diarrhea or other symptoms. The illnesses were detailed Wednesday in the New England Journal of Medicine.

Salmonella can cause diarrhea, fever and stomach pain but is rarely fatal. It is most dangerous to very young children, the elderly and people with weakened immune systems. The infection is usually contracted from food, but live animals can transmit it, too, because the bacteria can be in their feces.

Salmonella outbreaks have been linked to hatcheries for more than 50 years. And health officials have long warned that people can get salmonella from touching chickens — especially children, who tend to put their fingers in their mouths. Indeed, the CDC says children under 5 shouldn't be allowed to touch chickens at all.

Health officials also advise people not to bring birds into their homes and to wash their hands thoroughly after handling live poultry.

About 20 hatcheries mail live chicks overnight in the U.S., supplying not only feed stores and farms but amateurs with backyard coops. The mail-order houses have been seeing record sales in recent years.

"It's all part of this greener, healthier lifestyle," said Behravesh, a veterinary epidemiologist.

Jonah McDonald, a 32-yearold Atlanta man who keeps three hens and insists a backyard egg tastes better, said he does not know of anyone who has gotten salmonella from handling chickens.

"The kids in my neighborhood come over and feed scraps to my chickens," he said. "It's a real community thing."

The CDC described an eightyear investigation into salmonella illnesses, with more than 80 percent of the cases tied to a single hatchery in the western U.S. While CDC officials refused to identify the business, a previous report on the investigation by the health agency indicated it is in New Mexico.

Investigators interviewed victims and concluded many had caught salmonella from touching chicks or ducklings, often at home. From there, most of the illnesses were traced to the hatchery.

Behravesh said the hatchery has taken steps to curb the spread of salmonella — including replacing equipment, adopting new egg-cleaning procedures and vaccinating chickens — and is not considered a health threat. She said she was not aware of any fines or penalties against the business over the outbreak.

During the eight years studied, the annual number of illnesses linked to the hatchery ranged as high as 84, with 29 cases last year and only one so far in 2012