The Iowa Department of Public Health (IDPH) would not identify the woman, other than to say she is an adult between the age of 18 and 40 living in Northwest Iowa. IDPH Medical Director Dr. Patricia Quinlisk said the woman, who has since recovered, had been infected with a strain of listeria monocytogenes that matched the strain detected in cantaloupes from Jensen Farms in Colorado -- the place at the center of the investigation into the listeriosis outbreak.
The U.S. Centers for Disease Control and Prevention identified 100 cases associated with the outbreak in 20 states as of Monday morning, prior to Iowa's inclusion on the list. Nationwide, the case is the third known to have involved women who were pregnant when they fell ill. The CDC reported Tuesday that two other women were pregnant at the time they were infected and that their pregnancies were being monitored.
According to the CDC, pregnant women are about 20 times more likely than other healthy adults to get the bacterial infection, and roughly 17 percent of listeriosis cases occur during pregnancy.
"Pregnant women are much more susceptible to having symptoms and becoming severely ill from listeria," Quinlisk said. "Once they have the infection, the complications can be quite serious."
The CDC lists foods such as hot dogs, luncheon meats and certain cheeses as listeria risks for pregnant women. Cantaloupes are not generally considered to be unsafe. That, along with the fact that the incubation time of the illness can be lengthy, has confounded investigators.
"While Jensen Farms has issued a voluntary recall of its Rocky Ford-brand cantaloupes, and the recalled items should be off store shelves, more illness may be reported because it can take up to two months for a person to develop listeriosis after eating contaminated food," Quinlisk said in a press release issued Wednesday.
Last week, federal health officials said the outbreak is the deadliest foodborne disease in more than a decade.
"For the public, it's important to know that if you know the cantaloupe you have is not Jensen Farms, then it's OK to eat. But if you're in doubt, throw it out," Dr. Thomas Frieden, director of the CDC, said last Wednesday. "We do anticipate there will be a rising number of cases in the days and weeks to come."
Listeria can cause fever, stiffness in the neck, confusion and vomiting. The elderly and those with weakened immune systems are at a greater risk for serious symptoms. However, not everyone who is exposed to the bacteria develops illness, Quinlisk said -- a fact that makes tracking the source of such infections even more difficult for investigators.
Although there have other listeria outbreaks in recent years, this is the first one attributed to whole cantaloupes, according to the U.S. Food and Drug Administration.
ABC News' Brian Hartman and Christina Caron contributed to this report.