Which term best defines the primitive instinct for the body to defend itself?

This instinct is called the survival instict. 

It's a very basic instinct that most if not every organism has that forces it to try and survive in any case in order to be able to pass off it's genes and possibly reproduce.

You did not show us the image of the burger, but i shall try to answer as best i can.  when purchasing food, always consider the ingredients as well as the person who prepared it. consider if they were hygienic people or not, in case the burger was infected with bacteria. no mortal should place an infected burger into their mouth.   

Its simple its a because they are more fat