Types of viruses

 

File infector virus

These are generally memory resident. File infector viruses infect program files. Executable code files such as .com and .exe files are on their attack . These viruses are so much infectious that they can effect other files when an infected program is run from floppy, hard drive, or from the network. It means virus will stay in memory and continue to infect other programs. After memory becomes infected, any noninfected executable that runs becomes infected. Examples of known file infector viruses include Jerusalem and Cascade.

Boot sector viruses
 
 They infect the system area of a disk; that is, the boot record on floppy disks and hard disks.A boot sector virus does not need to be able to successfully boot the victims computer to infect it.  All floppy disks and hard disks (including disks containing only data) contain a small program in the boot record that is run when the computer starts up. Boot sector viruses attach themselves to this part of the disk and activate when the user attempts to start up from the infected disk. These viruses are always memory resident in nature. Most were written for DOS, but, all PCs, regardless of the operating system, are potential targets of this type of virus. All that is required to become infected is to attempt to start up your computer with an infected floppy disk Thereafter, while the virus remains in memory, all floppy disks that are not write protected will become infected when the floppy disk is accessed. Examples of boot sector viruses are Form, Disk Killer, Michelangelo, and Stoned.

Master boot record viruses
They are memory-resident viruses that infect disks in the same manner as boot sector viruses but differs as they  infect the MBR (Master Boot Record) instead of the boot sector. The difference between these two virus types is where the viral code is located. Master boot record infectors normally save a legitimate copy of the master boot record in an different location. Windows NT computers that become infected by either boot sector viruses or master boot sector viruses will not boot. This is due to the difference in how the operating system accesses its boot information, as compared to Windows 98/Me. If your Windows NT systems is formatted with FAT partitions you can usually remove the virus by booting to DOS and using antivirus software. If the boot partition is NTFS, the system must be recovered by using the three Windows NT Setup disks. Examples of master boot record infectors are NYB, AntiExe, and Unashamed.

Multipartite (also known as polypartite) viruses
They  infect both boot records and program files. These are particularly difficult to repair. If the boot area is cleaned, but the files are not, the boot area will be reinfected. The same holds true for cleaning infected files. If the virus is not removed from the boot area, any files that you have cleaned will be reinfected. Examples of multipartite viruses include One_Half, Emperor, Anthrax and Tequilla.

Macro viruses
 W97M.Melissa, WM.NiceDay and W97M.Groov are the examples of these kind of viruses. These types of viruses infect data files. They are the most common and have cost corporations the most money and time trying to repair. With the advent of Visual Basic in Microsoft's Office 97, a macro virus can be written that not only infects data files, but also can infect other files as well. Macro viruses infect Microsoft Office Word, Excel, PowerPoint and Access files. Newer strains are now turning up in other programs as well. All of these viruses use another program's internal programming language, which was created to allow users to automate certain tasks within that program. Because of the ease with which these viruses can be created, there are now thousands of them in circulation.