WEDNESDAY, Sept. 28 (HealthDay News) -- The ongoing outbreak of food-borne illness connected to listeria-tainted cantaloupes has now infected 72 people in 18 states and claimed 13 lives, U.S. health officials said Wednesday, making it the deadliest such outbreak in more than a decade.

The deaths have occurred in eight states, including two in Colorado, one in Kansas, one in Maryland, one in Missouri, one in Nebraska, four in New Mexico, one in Oklahoma, and two in Texas, according to the U.S. Centers for Disease Control and Prevention.

But the toll is expected to rise, as investigators continue to probe the causes of additional deaths.

"At this point we have definitively confirmed 72 cases and 13 deaths with laboratory-confirmed listeria, including two pregnant women who, so far as we know, are doing OK, both in terms of their own outcome and their fetus," CDC Director Dr. Thomas R. Frieden said at an afternoon news conference.

"This is the deadliest outbreak of a food-borne disease that we've identified in more than a decade," he added, and it's the 12th one this year.

Unlike other bacteria, listeria can flourish in colder temperatures. So, "if you've got a contaminated cantaloupe in your refrigerator, the listeria will continue to grow," Frieden said. "That's one of the reasons why we may see continued cases from cantaloupe already in people's refrigerators in the days and weeks ahead."

Added Dr. Margaret Hamburg, commissioner of the U.S. Food and Drug Administration, "We will see more cases likely through October because patients can develop this disease up to two months after eating contaminated food."

Although listeria tends to infect fewer people, it is typically deadlier than other food-borne pathogens and inordinately affects the elderly, newborns, pregnant women and anyone with a weakened immune system. People can develop meningitis from the organism, but many people only experience milder diarrhea.

According to the CDC, some 1,600 cases are reported annually in the United States, resulting in 260 deaths.

Listeria bacteria are also particularly dangerous because they can thrive at both room temperatures and refrigerator temperatures.

And "the incubation period can be quite long, as little as three days but up to two months," said Philip Alcabes, a professor in the School of Public Health at Hunter College in New York City.

The bacterium Listeria monocytogenes, which was first associated with food-borne illness in the 1980s, tends to grow in soil and water.

"The concern would be that the outside of cantaloupe is contaminated [and] when you slice into it, the knife can carry bacteria into the part that you eat," Alcabes explained.

But animals can also carry the organism and pass it on to humans through meats, dairy products and other foods of animal origins. Most listeria outbreaks are from animal products, not produce, the CDC said.

According to the Associated Press, 52 people died from an outbreak of listeria in soft cheese in 1985 and as many as 21 died from contaminated hot dogs and deli meats in 1998.

The current outbreak has been traced to Rocky Ford-brand cantaloupe grown at Jensen Farms in Holly, Colo. The company issued a voluntary recall of the produce earlier this month.

In addition to avoiding Rocky Ford cantaloupes from Jensen Farms, health authorities advise washing fruits and vegetables thoroughly before eating.

"Your grandmother told you to wash fruits and vegetables. It's probably not bad advice," Alcabes said.

The CDC's Frieden said that Jensen Farms has ended its harvest for the season, but he recommended that people throw out any cantaloupe unless they know for sure that the fruit was not grown there.

More information

The U.S. Centers for Disease Control and Prevention has more on listeria.