Types of Computer Virus - Viruses - Worms - Trojans

Virus removal and combating computer infections is now big business but in order to be successful, computer users need to know who the enemy is. Computer geeks make a distinction between crackers and hackers but to the normal user, both appear to be one of the same.

Crackers are classed as those who break into systems to do damage whereas hackers are those who enjoy taking programs apart just to see how they work. The original description of a hacker was somebody who made furniture with an axe. Like most computer viruses, this method of furniture making was very crude but nonetheless effective.

Viruses are in reality a certain type of computer malware, which is a common term covering all types of malicious software. The most common types of malware are viruses, worms and Trojans.


A Computer Virus is normally concealed inside another program such as an installation program delivered via an email attachment. The term Computer Virus came about because of the similarity with biological viruses that require a host organism to live and reproduce, When the host program is run, the Computer Virus program runs at the same time. Once the Computer Virus is in memory it is able to do its dirty work which normally includes infecting other programs on the host computer.

After the infection stage of the Computer Virus, there's a destructive stage. The Computer Virus waits for a pre-determined trigger (such as a specific date or a certain number of times the Computer Virus has replicated itself) before delivering its 'payload'. Payloads range from basic messages to file deletion commands to destruction of the core operating system.

When viruses were first developed they were commonly distributed on floppy disks. However, with the evolution of the Internet, downloaded files and email are now the preferred delivery mechanisms. email can also contain attachments which can be any type of computer file. Any executable file can be infected with a Computer Virus, and should never be run unless you are absolutely certain they are virus free.


Worms are very much the same to viruses in that they are self replicating. Unlike a Computer Virus a worm does not need another executable program to be distributed. They reproduce themselves across networks by email sending and without any human assistance.

Worms usually affect networks more than particular computers on the network. Because of their self replicating behavior, worms can overload network resources very quickly which in turn causes slowdowns in data transmission due to massive bandwidth consumption.

Worms can also be designed to carry a payload which can create a 'backdoor' on the infected computer. Two of the more infamous worms that did this were Sobig and Mydoom. A backdoor is a hidden access point to a computer system that effectively bypasses any normal login procedures. The backdoor allows access to the infected computer by spammers to send junk email from that system.


Trojans or Trojan Horses are the third common type of malware. A trojan is a program that pretends to do one thing but actually does something different. The term comes from the story of the Trojan Horse. During the siege of Troy, the Greeks left a large wooden horse outside the gates, supposedly as a peace offering. The Trojans took the horse inside the city walls only to find it was full of Greek soldiers who quickly overrun the city.

Trojans on a computer is very much the same. It looks like a harmless or useful program but in reality contains concealed code that can erase data, corrupt files, install backdoors and log keystrokes so that hackers can steal information such as credit card numbers and passwords.

Although frequently referred to as a type of Computer Virus, Trojans cannot replicate themselves like a Computer Virus or worm.. It is purely designed to gain access to your computer system and wreak havoc, just like the legendary Greek soldiers.