As we already know, computer viruses are such kinds of viruses which replicate themselves and have the capability to destroy the data and information contained in a computer. Nobody can give the exact number of viruses created till today because everyday a new virus is being created. Depending upon the nature of virus, their mode of infection, mode of replication and area of the disk where they reside, they can be classified into different categories.
Broadly, the computer viruses can be classified into the following types:
Time bomb: Time bomb viruses are the viruses which are set to be activated on a certain date. A famous example of time bomb is ‘Friday the 13th virus.’ This virus is activated on the 13th of a month which is a Friday and destroys all the data on this day.
Time bomb virus stops the working of a program. The virus contains information for damaging the particular sections of .COM or .EXE files at fixed dates as described in the programs. As the date matches, the program gets corrupted and simply stops working at all.
Logic bomb is another virus which destroys the data, in case logical operations are to be performed in the computer’s data processing or program generation. It has such instructions that on a fixed date, in a fixed section of a disk a fixed number of data are destroyed. Let’s say, on 13th of the month, on Friday, 1813 bytes are destroyed.
Trojan horse: Trojan horse viruses are such viruses which have the ability to attach themselves to programs but do not replicate. Normally they are used in computer networks where they take advantage of the access control enjoyed by the parent program and cause havoc. The user runs the program thinking it is a genuine one, not knowing that there is a virus hiding in it.
Trojan horse got its name from the wooden horse of Greek mythology. A program is hidden inside another apparently useful program. While the useful program is being executed, the hidden part of the program does something bad, like erasing your FAT and directory. Trojan horse does not affect if the software is purchased from reliable source which has a license. If the software is pirated, the hidden program is activated and the important part of the program is corrupted. As you go on copying the pirated software, the virus infection goes on increasing one day, when the program will totally stop working.
Worms: Worms are the viruses that do not destroy data but get into the computer memory and slow down the operating and processing speed of the computer. They reside on one side of the sectors or tracks of the disk just like worms residing in the host bodies. The virus corrupts and interrupts the processing instructions required for the proper execution of the programs and slows down the whole system.