A swat can't save you.

By Meghan Overdeep

Flies look innocent enough, buzzing around from place to place, going about their business. But don't let them fool you. There's nothing innocuous about what happens when these winged invaders touch down on your food.

As Dana Nayduch, a molecular biologist at the US Department of Agriculture, explained to Tonic, the nastiness begins the moment a fly sets food on your food. That's right. By the time you go to swat it away, the damage has likely already been done.

As soon as a fly lands on a piece food it begins exploring, determining whether or not it's worth eating. As they scuttle around, sometimes defecating, germs from wherever they touched down previously (a toilet or a dumpster, for example) get transferred from their bodies to your food. Once a fly has decided whether or not something is worthy of their consumption, it will squirt regurgitated saliva from a snout-like mouth to liquefy it. Why you ask? Because they don't have teeth.

Once they've liquified the food with their vomit, it can be slurped up and swallowed. This entire process begins minutes or seconds after it lands.

Now, before you swear off picnics for life, here's a little perspective. Unless you're in a particularly germ-infested area or the flies enjoyed an extended period of time with the food, Nayduch explained to Tonic that you're most likely safe from life-threatening illnesses. That's what our immune systems are for.

"Every day you touch doorknobs, money, and credit cards, and then you bite your nails or rub your eye and inoculate yourself with bacteria, but our immune system takes care of it," Nayduch said. "So, flies pose very little threat in a country like ours that has good sanitation and where filth is kept sequestered."

That being said, flies have been responsible for transmitting up to 65 diseases to humans including cholera, typhoid fever, tuberculosis, and leprosy. So next time you see a fly touch down on your lunch, you might want to think twice before you eat it.