"That long incubation period is a real problem," said Robert Tauxe of the Centers for Disease Control. "People who ate a contaminated food two weeks ago or even a week ago could still be falling sick weeks later."
So just because someone seems fine a day after trying a sample doesn't prove it's safe.
No cases had been reported in Pennsylvania or New Jersey as of Tuesday, but they were among the 26 states were the Jensen Farms melons were distributed, according to the FDA.
Consumers should look for a label saying "Colorado Grown," "Distributed by Frontera Produce," "Jensenfarms.com" or "Sweet Rocky Fords," according to the FDA. If there's no label, ask the store that sold it, the CDC suggests.
Anyone with such a recalled melon should discard it and sanitize any surfaces it contacted.
Confirmed were deaths in Maryland, Missouri, Kansas, Nebraska, Oklahoma, Texas and New Mexico. Additional fatalities were being investigated in New Mexico, Kansas and Wyoming.
The second worst U.S. food-related outbreak involved hot dogs and deli meats in 1998-99, in which listeria was blamed for at least 14 deaths.
Overall, the number of new cases, including the deaths, was up to 72, including 10 other states: Virginia, West Virginia, Indiana, Illinois, Wisconsin, Florida, Nebraska, Montana, North Dakota and California.
Not reporting cases, but also receiving Jensen Farms cantaloupes from July 29 through Sept. 10 were New York, North Carolina, Ohio, Tennessee, Minnesota, Arkansas, South Dakota, Idaho, Arizona and Utah.
To prevent listeria, consumers are advised to "thoroughly cook raw food from animal sources" and "rinse raw vegetables thoroughly under running tap water before eating," according to the CDC.
For more tips and information, go to: http://go.philly.com/listeria.
Contact staff writer Peter Mucha at 215-854-4342 or firstname.lastname@example.org.
The Associated Press contributed to this article.