An antivirus scanner can identify the program s signature and warn the user. An encrypted virus makes things a bit more complex. By scrambling the signature, encryption makes it impossible for antivirus software to spot it. Instead, the scanner must look for a decryption routine that the virus uses. These viruses are fairly easy to detect.
Polymorphic viruses raise the bar again by encrypting both the virus and the decryption routine.
Every infection looks a bit different from every other infection. Companies that write antivirus software use generic decryption routines to find polymorphic viruses. Macro viruses have become increasingly popular because they re easy to create and, because programs such as Microsoft Word are in such widespread use, macro viruses spread quickly. They generally infect documents and templates. Trojan horse programs Security experts are most concerned about trojan horse programs.