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In the weeks before the recent Listeria outbreak, thousands of Americans were likely exposed to bacteria-tainted cantaloupes, but most did not get sick.

NPR's Allison Aubrey explains how healthy stomachs and quick immune responses help us fight off even the nastiest pathogens.

ALLISON AUBREY, BYLINE: With more than 20 deaths linked to contaminated cantaloupes and about 100 cases of serious illness, lots of us may have been wondering - is it possible that I ate some of these bad cantaloupes?

Food scientist Don Schaffner, of Rutgers University, says it's actually quite possible.

DON SCHAFFNER: Probably a lot of people ate this cantaloupe. A lot of people probably ate a lot of Listeria on the cantaloupe.

AUBREY: But did they all get sick? Well, in most cases, Schaffner says, not really, certainly not seriously ill, even though we're talking about some virulent strains of a pretty common bacteria.

SCHAFFNER: Maybe they had an upset stomach. Maybe they didn't feel so good for a day or two, but it was certainly was nothing worth going to the doctor about. And they probably recovered and they were fine.

AUBREY: The median age of the cantaloupe victims who were hospitalized was 79, close to 80 years old, as well as a few pregnant women. So what is it about younger people that gives them an advantage in fighting off food-borne pathogens? Well, for starters, experts point to what's happening in our stomachs. Ideally, we have a lot of acid in our stomachs. And when cells of bacteria enter the stomach, they don't find the environment too hospitable.

SCHAFFNER: The bacteria come in, in many cases, they'll die in the stomach.

AUBREY: But this doesn't always happen when acid in the stomach is altered. For instance, when people take medicines to reduce acid reflux, well, studies show they seem to become more susceptible to stomach bugs.

SCHAFFNER: We do know that people that eat a lot of antacids or who are taking proton pump inhibitors are certainly at higher risk of food poisoning.

AUBREY: But when everything is functioning optimally, immunologists say the first and best line of defense against harmful pathogens really is the stomach.

PROFESSOR ROSS KEDL: Its primary purpose is to help us digest things. But it just so happens to work very, very well as a barrier against microbes getting further down into our gut.

AUBREY: Ross Kedl is an associate professor of immunology at National Jewish Health in Denver. And he says when bacteria do survive the stomach and make it down into our intestines, there is still another opportunity for our bodies to mount a quick response and fight them off. Basically, what happens is that our immune system sends white blood cells to the scene, and each one acts as a little foot soldier, surrounding and battling it out with the Listeria cells.

KEDL: The job of that white blood cell is to turn itself into its own killing machine, to help grind up the bacteria that's inside of it.

AUBREY: And Kedl says since younger people's immune systems are typically more efficient at mounting this response, they tend to have an advantage.

KEDL: So that is an important checkpoint, the ability to block it from making that invasion in the first step. That ability drops as you get older, as well.

AUBREY: Kedl's colleague, Dr. Erwin Gelfand, also of National Jewish, says there are many factors that may increase a person's susceptibility to food-borne illness. For instance, when someone is fighting off the common cold. Well, they may not be able to handle the attack of pathogens since viruses can alter the way the immune system functions. And then there's conditions such as vitamin deficiencies or even stress - they're known to alter immunity, as well.

Finally, there's the business of eating habits. Rutger's Don Schaffner says there's one favor you can do yourself when you eat fruit such as cantaloupe. Once you've bought it and sliced it, don't wait too long to eat it.

SCHAFFNER: Use it up within a couple of days. Don't let it hang around. One of the unique things about Listeria is that even in the refrigerator, the organism will grow and multiply slowly.

AUBREY: There's no way to completely eliminate the risk of food-borne illnesses. But understanding how best to battle back against the bacteria can't hurt.

Allison Aubrey, NPR News.

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