Macro viruses – These are the largest of the three virus types. They use built-in programming scripts in such applications as Microsoft Excel and Microsoft Word to automate the tasks. When an unsuspecting user opens a document infected by a macro virus, they give it the green light to make copies of itself in other documents and multiply.
Boot record infectors – These viruses are known also as boot viruses or system viruses. They attack programs stored on bootable floppy or hard disks. Every time a user boots up, the virus-infected code automatically runs as well. This gives hackers full control of an entire system and lets them do exactly what they want with it. Boot record infectors can be spread from one infected bootable disk to another.
File infectors – These viruses target .EXE or .COM files and changes them. File infectors get activated every time users run a compromised program. What makes this type of virus especially insidious is that it can simply stay inactive in the memory until it gets a cue to infect other program files. These viruses can get transmitted over networks, the Internet or by using infected disks.