<P> Polymorphic code was the first technique that posed a serious threat to virus scanners . Just like regular encrypted viruses, a polymorphic virus infects files with an encrypted copy of itself, which is decoded by a decryption module . In the case of polymorphic viruses, however, this decryption module is also modified on each infection . A well - written polymorphic virus therefore has no parts which remain identical between infections, making it very difficult to detect directly using "signatures". Antivirus software can detect it by decrypting the viruses using an emulator, or by statistical pattern analysis of the encrypted virus body . To enable polymorphic code, the virus has to have a polymorphic engine (also called "mutating engine" or "mutation engine") somewhere in its encrypted body . See polymorphic code for technical detail on how such engines operate . </P> <P> Some viruses employ polymorphic code in a way that constrains the mutation rate of the virus significantly . For example, a virus can be programmed to mutate only slightly over time, or it can be programmed to refrain from mutating when it infects a file on a computer that already contains copies of the virus . The advantage of using such slow polymorphic code is that it makes it more difficult for antivirus professionals and investigators to obtain representative samples of the virus, because "bait" files that are infected in one run will typically contain identical or similar samples of the virus . This will make it more likely that the detection by the virus scanner will be unreliable, and that some instances of the virus may be able to avoid detection . </P> <P> To avoid being detected by emulation, some viruses rewrite themselves completely each time they are to infect new executables . Viruses that utilize this technique are said to be in metamorphic code . To enable metamorphism, a "metamorphic engine" is needed . A metamorphic virus is usually very large and complex . For example, W32 / Simile consisted of over 14,000 lines of assembly language code, 90% of which is part of the metamorphic engine . </P> <P> As software is often designed with security features to prevent unauthorized use of system resources, many viruses must exploit and manipulate security bugs, which are security defects in a system or application software, to spread themselves and infect other computers . Software development strategies that produce large numbers of "bugs" will generally also produce potential exploitable "holes" or "entrances" for the virus . </P>

Write any five names of viruses that has infected your system