Which hygiene step would best prevent a staph infection?

Cleanliness/washing.
(Not exactly sure what a more professional term for the "step" is...)
But to wash out open wounds in the skin kills the bacteria that has already gone inside, as well as bacteria on the skin that can very easily make its way into the wound.

This also goes for internal infections, not only on the skin.
For example, drinking a lot of water, eating a lot of vitamin/nutrient/antioxidant rich foods can cleanse your blood, and help your body. It will make it stronger as well, making you less prone to infections, various diseases, and getting sick.

Hope this helped. :)

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