(NBC News) - More than 600 people in 45 states have caught salmonella from backyard chickens and ducks, federal health officials reported Tuesday.

The infection hasn't killed anyone but it's made 138 sick enough to be hospitalized, the Centers for Disease Control and Prevention said.

"The investigations this year involve the largest number of sick people linked to live poultry that we've seen, with the second highest being in 2013 at 579 illnesses," the CDC said in a statement. CDC counted eight separate outbreaks starting in January.

"These outbreaks serve as a reminder that all live poultry can carry Salmonella bacteria, even if they look healthy and clean," the statement added.

Salmonella lives inside the digestive tracts of birds and it spreads in their droppings. Chickens, especially, walk through their droppings as they scratch for food and roost. The germs can live in the birds' feathers and then spread in the places where the chickens walk and roost. People can be infected if they kiss or otherwise handle the birds.

"In the eight outbreaks, 611 people infected with the outbreak strains of Salmonella were reported from 45 states," the CDC said. About a third of cases were kids 5 and younger.

"These outbreaks are expected to continue for the next several months since flock owners might be unaware of the risk of Salmonella infection from live poultry or participate in risky behaviors that can result in infection," the CDC said.

"All people reported purchasing live baby poultry from several suppliers, including feed supply stores, Internet sites, hatcheries, and friends in multiple states," it added. "Ill people reported purchasing live poultry to produce eggs, learn about agriculture, have as a hobby, enjoy for fun, keep as pets, or to give as Easter gifts. Some of the places ill people reported contact with live poultry include their home, someone else's home, work, or school settings."

The CDC says it counted 2,200 salmonella infections tied to live poultry between 1990 and 2013, including 306 people who were hospitalized and five who died. The agency estimates that for every salmonella infection they know about, there are 30 others that are never reported.

"Always wash your hands thoroughly with soap and water right after touching live poultry or anything in the area where they live and roam," CDC advises. "Do not let live poultry inside the house."

Salmonella can cause severe diarrhea as well as fever and stomach cramps.