A technique of some anti-virus programs to store information about files in order to notify the user about file changes. Internal vaccines store the information within the file itself, while external vaccines use another file to verify the original for possible changes.
by means of this technique, the antivirus program stores information on files. Whenever it detects a difference between the information it has stored and the current file data, it will warn you of the situation. There are two types of vaccine: Internal (the information is stored within the file itself, which means that when it is executed it automatically checks for any changes) and External (information is stored in a specially created file, which the program uses to check against the original for possible changes).