Live poultry in backyard flocks are linked to several multistate outbreaks of salmonella infections that have now sickened 212 people in 44 states, federal health officials warned Monday.

The most recent illnesses began on June 21, with 88 of the cases reported in the last month, the Centers for Disease Control and Prevention, or CDC, said in an advisory. The federal agency is working with multiple states in investigating several outbreaks of salmonella infections linked to contact with live poultry in backyard flocks, it said.

A half-dozen strains of salmonella bacteria have sickened people starting in the middle of February, with 34 of the 212 sickened people hospitalized. More than a quarter of those stricken were under the age of 5, according to the CDC. Young children are particularly vulnerable because their immune systems are still developing, and they're more likely to put items like fingers and pacifiers in their mouths.

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The six states so far not affected are North Dakota, South Dakota, Virginia, New Jersey, Maryland and Delaware. The District of Columbia was also spared.

People infected with the outbreak strains of Salmonella, by state of residence, as of July 13, 2018 (n=212) Centers for Disease Control and Prevention

Chicks and ducklings from multiple hatcheries have been tied to the outbreaks, and many of the people stricken reported getting chicks and ducklings from sources including feed supply stores, websites, hatcheries and relatives, said the agency. It added that some of the infections have proven difficult to treat with common antibiotics, requiring another type to be used.

Birds can carry salmonella bacteria even though they may appear healthy and clean, and people can be infected by touching live poultry or objects in their environment, the health agency cautioned. "Even handling baby birds displayed at stores can cause a salmonella infection," the CDC said.

Health officials advise washing with soap and water right after touching live poultry or anything nearby. They also advise not letting live birds inside the house, especially where food or drink is prepared, served or stored. Also, no kissing or snuggling with birds only to then touch your face or mouth.

Raising chickens in urban areas is a growing trend, with Las Vegas one of the latest cities to approve residential coops. A study of 50 of the most populated U.S. cities found 93 percent allow backyard flocks. In 2013, almost 1 percent of all U.S. households surveyed by the U.S. Department of Agriculture reported owning backyard fowl, and 4 percent more planned to start in the next five years.