This virus hides from the operating system when the system checks the location where the virus resides, by forging results that would be anticipated from an uninfected system. The different kinds of virus, one of them known as fast-infector virus infects not only programs that are executed but also those that are merely accessed therefore running antiviral scanning software on a computer infected by such a virus can infect every program on the computer. Another kind called the slow-infector virus infects files only while they are modified, so that the modification appears legitimate to checksumming software. Yet another kind called the sparse-infector virus infects only on certain occasionsâ€for example, it may infect every tenth program executed. This strategy makes it more difficult to detect the virus.