A Virus is defined as a program inserted into another program. It gets activated by its host program. It replicates itself and spreads to others through floppy transfer. A virus infects data or program every time the user runs the infected program and the virus takes advantages and replicates itself.

There are two types of computer viruses ‘parasitic’ and ‘boot’ virus.

1. A Parasitic virus attaches itself to other programs and is activated when the host program is executed. It tries to get attached to more programs so that chances of getting activated is more. It spreads to other computers when the affected programs are copied. ‘Jerusalem’ and ‘Datacrime’ are parasitic viruses.

2. A Boot virus is designed to enter the boot sector of a floppy disc. It works by replacing the first sector on the disc with part of itself. It hides the rest of itself elsewhere on the disc, with a copy of the first sector. The virus is loaded by the computers built-in start-up program when the machine is switched on. The virus loads, installs itself, hides the rest of itself and then loads the original program. On a hard disc, virus can occupy DOS boot sector or master boot sector.

Some Reported Viruses

3. C-Brain: Amjad and Basit, two pakistani brothers, developed this software in January 1986 to discourage people from buying illegal software at throwaway prices. This was the most famous virus ever found and has a record of damaging few millions of personal computers. This is designed to stay in the boot sector of the disc or near zero sector. The virus enters the machine memory once the PC is booted with the infected floppy.

4. Macmag: This virus attacked Apple Macintosh computers only. Not much damage is reported because of this virus. This was not noticed on any IBM compatible PCs. It displayed a message of peace on the monitor and killed itself.

5. Cascade: This virus attacked IBM PCs and compatibles. The letters on the screen could be seen dropping vertically down to the bottom of screen after the virus picked them off in alphabetical order. This is a sort of parasitic virus. It attaches itself to other programs and gets activated when the host program is executed. It gets copied to other PCs when the programs are copied.

6. Jerusalem: Found in 1987 at Hebrew University, Jerusalem, this virus was designed to activate only on Friday, January 13 and delete all the files executed on that day. This infects the COM and EXE files. This is similar to Cascade virus in that it is parasitic in nature. This virus attaches itself to COM and EXE files to damage the data.

7. Daracrime/Columbus or October the 13th virus: This virus is similar to Jerusalem and was programmed to attack on October 13, 1989. Track zero of computer hard disk is destroyed and the contents of discs are rendered unreadable. This virus enters COM and EXE files and damages the hard disk. An antidote called ‘Vchecker’ was developed by the American Computer Society. Fortunately the virus was located in March 1989 itself and the damage reported after October 13 was minimal.

8. Bomb: This is also know as ‘Logic Bomb’ and ‘Time Bomb’. An event triggered routine in a program that causes a program to crash is defined as a ‘bomb’. Generally, ‘bomb’ is a software inserted in a program by a person working in the company.


Some types of anti virus are;

1.Microsoft Security Essentials: Released by Microsoft in late 2009, Microsoft Security Essentials sports more than a typically verbose Microsoft name: it’s also a really good antivirus. Lightweight enough to run on older machines without crippling their performance, yet competent enough to handle most viruses and malware out there.

2. AVG: Anti Virus Guard has become synonymous with free anti-virus, and there’s a reason for this: AVG offers complete malware protection, with considerably less bloat than the top pay-to-use antivirus clients. And while AVG Free does constantly remind you that you could pay for the professional version of the program, it does this without ever getting in the way of the program’s core purpose: protecting you from viruses.

3. Malwarebytes: This program doesn’t run in your system background and constantly protect you, but when you run into a problem running Malwarebytes will usually take care of what other programs can’t.

4. AVAST: This is one of the top free anti-viruses on the market, and for good reason: it’s remarkably complete. Expect great all-around protection, including against trojans and spyware. You can also expect constant reminders that there’s a free version you can upgrade to, on your desktop and in your inbox. Still, the protection is solid.

5. Comodo Firewall + Antivirus: Comodo is best known for its free firewall, but it also offers a bundled firewall and antivirus program. While the Comodo firewall isn’t the easiest to use, and the antivirus doesn’t include protection for non-virus forms of  malware, this one’s worth mentioning if you’re looking for a free securitysuite which includes both a firewall and anti-virus protection.