If you already found the typical house fly or blowfly to be icky, well, get ready to find them a lot more gross. A new study has determined that DNA analysis of these flies shows that they carry hundreds of different bacteria, including those that can cause diseases like blood poisoning, pneumonia, and stomach bugs.

Flies are widely known for their ability to spread diseases carried on their wings, feet, and legs, but while we always knew they were behind the spread of some pathogens, this study shows the true extent of how much they contaminate. Researchers used DNA sequencing to examine the microbes on the house fly and blow fly.

The blowfly had 316 types of bacteria, while the house fly had 351. The two species often carried the same type of bacteria. These flies are often exposed to harmful diseases because they dine on carrion and feces, moving from one place to the next and bringing substances from those places along with them.

“We believe that this may show a mechanism for pathogen transmission that has been overlooked by public health officials, and flies may contribute to the rapid transmission of pathogens in outbreak situations,” said Donald Bryant, Ernest C. Pollard Professor of Biotechnology and professor of biochemistry and molecular biology, Penn State.