The question:

Is it true that flies vomit when they land on your food and you can get sick from it?

The science:

That fly buzzing around your picnic food may seem innocuous, but keep this in mind: It might have been snacking on spoiled garbage, roadkill or animal feces moments before landing on your meal.

Non-biting flies such as houseflies cannot chew their food because they have mouths akin to spongy straws. The small, winged critters land on a food source, regurgitate a fluid containing digestive enzymes to break it down, then slurp it back up.

Not only do flies vomit on our food -- assuming they're planning to eat it -- but that vomit may also contain pathogens they picked up from an earlier meal, including bacteria such as campylobacter, Escherichia coli or salmonella; parasites; and viruses, research shows.

Also, a housefly can carry more than 1 million bacteria on its body, including its head, six legs and feet, which have claws and sticky suction pads on the bottom.

This is more of a risk in food processing facilities such as poultry farms where large numbers of flies may gather, feast on chicken feces and transmit pathogens to eggs that end up on grocery store shelves, said Erika Machtinger, an associate professor of entomology at Pennsylvania State University. "That's our main concern with food safety," she said.

There is even legislation that requires certain methods to monitor and manage fly populations in poultry houses.

What else you should know:

To cause illness, flies would have to transmit what experts call an infectious dose of pathogens, or the dose required to make people sick, said Lyric Bartholomay, an expert on insect-borne diseases. The number of organisms needed to make a person sick depends on the pathogen. For instance, with shigella, a gastrointestinal bacteria, it may take 10 organisms to cause illness, whereas with salmonella, it would take more than 50,000, research shows.

"It's gross to learn about these things and to think about them, but I think there would be a lot more people a lot more ill if flies were transmitting high doses of these pathogens," said Bartholomay, director of the Midwest Center of Excellence for Vector-Borne Disease.

A fly or two are unlikely to pose a significant food safety risk. For instance, of the bacteria they carry, only "some small fraction" may cause disease, Bartholomay said.

But when flies land on food, vomit and defecate, and the food is not consumed for a while, "the higher the likelihood that these pathogens multiply and then health risks may increase," Machtinger said.

Experts recommend these precautions:

Limit the number of flies in your home by using screens when windows or doors are open and taking out garbage or any other spoilage that may attract them.Prepare for outdoor barbecues or picnics by placing traps such as flypaper, which attracts flies and kills them when they stick to it.Keep food that you're not actively eating covered, indoors or outdoors.Shoo away flies that try to land on your plate.The bottom line:

Flies do vomit when they land on food that they're preparing to eat, and that vomit can contain pathogens that can make you sick. But one or two flies buzzing around your plate are not likely to cause disease.