Boot viruses normally operate by supplanting the existing Master Boot Record located in the Boot Sector with its own viral code. This code is then read into memory every time the computer boots up, allowing the virus to deliver its payload while continuing to infect other disks that come into contact with the computer. Program viruses attach themselves to an application or document file. While in memory the program virus can deliver its payload and infect other executable or document files. Computer viruses exhibit a variety of characteristics that make them difficult to detect and remove.
Many viruses take steps to avoid detection.
Particularly insidious are polymorphic viruses. Polymorphic viruses are programs that are designed to change their code to look different throughout the life of the virus. These viruses are remarkable difficult to detect and remove. Finally, most viruses activate based upon some triggered event. A particular date, date range, time, or keyboard command will set the virus into motion.