Elizabeth Weise writes in USA Today an outbreak of salmonella in five Eastern states has sickened 42 people so far this year, with two hospitalizations. Dozens more might have been struck down were it not for a strikingly successful new tool used by public health officials to quickly figure out what was making all those people sick: the lowly shopper-loyalty card.

Food safety officials are increasingly finding value in plumbing shoppers’ food buying habits through these loyalty cards when they’re faced with foodborne illness outbreaks across communities and even states that seem to have no obvious links.

"It’s very helpful because it’s very hard for people to remember what they ate a couple of days before, not to mention a couple of weeks ago," says Casey Barton Behravesh, with the Centers for Disease Control and Prevention.

Supermarket loyalty-card programs were introduced in 1987. By the 1990s, they were widely used. In return for discounts on some items, they allow companies to track shopping habits. For epidemiologists, who study disease outbreaks, they’re a complete record of everything shoppers bought at the store going back for years.

Privacy is a huge concern in using cards to track food-borne illness outbreaks, officials say. All health departments are required to get permission to use them, Hammond says. "This is voluntary: People are not required to consent to having the grocery chain release their shopper-card history," he says.

In the salmonella outbreak among Eastern states, New York state and local health officials noticed an increased number of salmonella cases and started conducting routine interviews.

When they realized that all the patients shopped at Wegmans, a local supermarket chain, it was a "red flag," Behravesh says. Given permission by patients to check their shopper club card data, officials found "a lot of these people were buying bulk Turkish pine nuts," or foods that contained them, Behravesh says.

Other recent cases include:
•An outbreak of E. coli O157:H7 that sickened 33 people and led to 15 hospitalizations in five Western states in 2010 was quickly traced to raw milk Gouda cheese produced by Bravo Farms in Traver, Calif., using Costco purchasing data.

•A puzzling outbreak of salmonella Montevideo that sickened 272 people in 44 states in 2009 was finally cracked when health officials examined shopper records from Costco and saw that almost everyone who had gotten sick had purchased salami from Daniele Inc. Testing showed it was not the sausage but the black and red pepper it was coated in that carried the bacteria.

The shopper loyalty cards also can help public health workers when consumers misremember what they ate. "One person swore she didn’t eat cantaloupe, she only ate honeydew melons," says William Keene, a senior epidemiologist with Oregon Public Health Services. "When we pulled her records, we found that she only bought cantaloupe, not honeydew. When we showed her that, she said ‘Oh, I guess I did eat cantaloupe.’ "