Multipartite viruses: Multipartite viruses make use of a group of techniques that include infecting documents, executable, and boot sectors in order to infect the computers. Mostly, multipartite viruses first enter the memory and then infect the boot sector of the hard drive. Once this virus enters into the memory, it can infect the entire system. Stealth viruses: These viruses hide themselves by actually tampering with the operating system. It makes the antivirus packages believe that everything is functioning normally. Polymorphic viruses: These viruses have the ability to change their own signature at the time of infection. These viruses are very complicated and are difficult to detect. Polymorphic viruses cannot be detected by the signature based antivirus. Encrypted viruses: These viruses use cryptographic techniques to avoid detection. These viruses are quite similar to the polymorphic viruses in their outward appearance. Each infected system has a virus with a different signature. However, these viruses alter the way they are stored on the disk. They do not produce the modified signatures by changing their codes.