Biological creatures are completely open execution environments. Any compatible code we receive will be executed. Viruses are not alive, but simply bits of code that can get into our biological system and begin to execute. Our biological defense against unwanted execution comes in the form of physical barriers and our immune system, which uses heuristics to identify and destroy unwanted code. All execution, interception, and mitigation of the spread of malicious code (information) in our biology is done at a hyperlocal level.

There is something interesting in how these biological information systems differ from digital information systems. Thinking of securing an information system with physical barriers seems absurd, but that's how life literally works.