Still another type is infector is the memory resident infector. These virii, when run, go resident into memory and then infect other files when they are run or opened. These are among the most advanced virii around. There are special types of memory resident virii that include stealth, boot sector, and kernel infectors. Polymorphism and Encryption are too additional methods by which a virus can escape detection.
Encryption involves the viral code being either encrypted, or modified, by a set pattern or patters.
When run the viral code decrypts itself. Polymorphism is an advanced method by which a virus can encrypt itself using, for all practical purposes, an unlimited number of encryption patterns, which means that the virus is almost never the same twice. This is just a very broad and general overview, and is really a very schitty text when it comes to discussing what virii are.