Viruses like hepatitis a and norovirus come from human intestines. one of the easiest and most effective ways to prevent spreading them is

giving employees time off when they ask for it
good, proper hand washing
separate bathrooms for employees and customers
wearing clean uniforms to work

Good and proper handwashing is your answer.

While this may be somewhat disgusting Hepatitis A and Norovirus are diseases that spread through fecal matter contact with the oral cavity. This means that someone wasn't washing their hands after taking a dump, and so you have the viruses because they touched something you ate.

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