The news might not come as a surprise to anyone who's batted houseflies away from a lunch while it's waiting on the kitchen counter. But it shows that behaviour is scientifically verified, and that it's a good idea to keep them away.
The researchers warn not to eat food that has been sitting out, or to eat picnics in busy urban environments that might encourage flies to land on their food.
They have also mapped the exact microbiomes on 116 houseflies and blowflies from three different continents, allowing them to find out just what diseases they might be spreading.
"We believe that this may show a mechanism for pathogen transmission that has been overlooked by public health officials," said Donald Bryant, Ernest C Pollard Professor of Biotechnology at Penn State University.
Researchers also investigated the microbes on individual fly body parts including legs and wings, according to Stephan Schuster, research director at Nanyang Technological University, Singapore.
A fly's legs appear to transfer the most microbial content from one surface to another, he said.
Flies probably pick up the bacteria from faeces and decaying organic matter which they use to nurture their young, the study indicated.
Scientists found 15 instances of the human pathogen Helicobacter pylori - which causes ulcers in the human gut - on Brazilian blowflies.
"It will really make you think twice about eating that potato salad that's been sitting out at your next picnic," Mr Bryant said.
"It might be better to have that picnic in the woods, far away from urban environments, not a central park."
The study did suggest, however, that flies could help human society by serving as living "drones" or acting as early warning systems for the disease.
More about: #science