Why do many tourists  end sick once on holiday?

How can travellers protect themselves?

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1) In my opinion, it happens 'cos they don't know some features of national dishes in the country they went to. 
2) First of all, they should ask about products in this dish, if they learn there is a product they've never eaten, they shouldn't try it. Travellers ought to eat in hotels or in big restarants but not in a small local caffee.