Parasitic viruses attach themselves to programs, also known as executables. When a user launches a program that has a parasitic virus, the virus is surreptitiously launched first. To cloak its presence from the user, the virus then triggers the original program to open. The parasitic virus, because the operating system understands it to be part of the program, is given the same rights as the program to which the virus is attached. These rights allow the virus to replicate, install itself into memory, or release its payload. In the absence of anti-virus software, only the payload might raise the normal user's suspicions. A famous parasitic virus called Jerusalem has a payload of slowing down the system and eventually deleting every program the user launches.