VIRAL STRATEGIES
The authors of viruses have several strategies to circumvent antivirus software and to propagate their creations more effectively. So-called polymorphic viruses make variations in the copies of themselves to elude detection by scanning software. A stealth virus hides from the operating system when the system checks the location where the virus resides, by forging results that would be expected from an uninfected system. A so-called fast- infector virus infects not only programs that are executed but also those that are merely accessed. As a result, running antiviral scanning software on a computer infected by such a virus can infect every program on the computer. A so-called slow-infector virus infects files only when the files are modified, so that it appears to checksumming software that the modification was legitimate. A so-called sparse-infector virus infects only on certain occasions—for example, it may infect every tenth program executed. This strategy makes it more difficult to detect the virus.
By using combinations of several virus-writing methods, virus authors can create more complex new viruses. Many virus authors also tend to use new technologies when they appear. The antivirus industry must move rapidly to change their antiviral software and eliminate the outbreak of such new viruses.