Norovirus is commonly caused by people touching food with bare hands without having washed them or by contaminated surfaces on which food is prepared, according to a law firm that deals with cases of food-poisoning.

Source: https://www.postandcourier.com/columbia/business/in-sc-restaurants-with-repeated-health-code-violations-skirt-by-with-few-consequences/article_e0132246-5cbb-11ee-9d63-9bbbe59aa80b.html