Here are some definitions to make things a little clearer for you. Self-encrypting viruses encrypt these text strings differently with each infection to avoid detection. Signature - A search pattern, often a simple string of characters or bytes, expected to be found in every instance of a particular virus. Usually, different viruses have different signatures. Anti-virus scanners use signatures to locate specific viruses.
Some polymorphic virus use different encryption schemes and requires different decryption routines.
Thus, the same virus may look completely different on different systems or even within different files. Other polymorphic viruses vary instruction sequences and use false commands in the attempt to thwart anti-virus software. Macro Virus - A macro virus is a malicious macro. When a document or template containing the macro virus is opened in the target application, the virus runs, does its damage and copies itself into other documents. Continual use of the program results in the spread of the virus.