How can houseflies spread bacteria such as E. coli and Salmonella to food?

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Diagram showing the West Nile virus transmission cycle: a mosquito bites an infected bird (the primary reservoir host), then transmits the virus to humans or horses (incidental hosts) through subsequent bites. — infectious disease Birds are the primary reservoirs of West Nile virus. The virus is transmitted to humans and horses by mosquitoes that have bitten infected birds.

Houseflies (Musca domestica) act as mechanical vectors for infectious diseases by passively transporting pathogens from contaminated environments to human food sources.1

Because houseflies frequently move between decomposing organic waste, feces, carrion, and human food or drink, they easily pick up millions of microorganisms on their bodies and feet. When a fly lands on food, it deposits these bacteria—such as E. coli or Salmonella—directly onto the surface. Because the pathogen does not need to develop or replicate inside the fly to be transmitted, this process is considered a form of mechanical transmission. Consuming food contaminated in this manner can lead to various intestinal illnesses.231