So what interprets the virii code?
The operating system. The operating system executes the code. It has no reason to suspect the code. Remember, a virus is STILL an executable bitstream. All it does is find means to get into memory. Once it gets there, the operating system will find it, and execute it. An operating system works like this:
Code:
dim x as Long Integer
dim programArray Array[] //A resizeable array
x = 0;
while MEMORY.location != '/0' //the null bit
programArray[x] = MEMORY[x];
run programArray[x];
x++; //increment X by 1, in other words, move to the next memory location that is not empty
endWhile
So all the operating system does is find all executable code in memory, and RUN it. The operating system is almost like a virus, in that it is a constantly running loop. But UNLIKE a virus, the operating system has an exit condition (not shown above).
Without getting too technical, it's easy for you to see why once the virus gets into memory, your operating system will detect it as executable code, and RUN it. Again, all the virus needs to do is get into memory. How it gets there, is irrelevant.