This category is the most frequent threat that causes a big headache to us all. Malicious code includes, among others, viruses, Trojan horses, worms, backdoors and logic bombs.

Viruses

The Internet has served as a breeding ground for the widespread propagation of viruses. Viruses represent a severe security threat that can damage the status of a company. Decreased productivity, altered files, lost data and unintended disclosures are some of the many problems viruses can cause.
 
A virus is an executable piece of code that, when executed, replicates and attacks other objects, such as other programs or data files. Viruses are more dangerous in a network environment, where they can propagate rapidly on the machines connected to the network, infecting everything on their way and causing destruction.

How does a virus first appear on your computer? All you need to do is run a single infected program that you downloaded from the Internet, or you transferrred from a floppy disk or a USB that your friend gave you to copy his favorite game.

Trojan horse

The Trojan horse is named for the Greek myth of the giant wooden horse that Greek solders gave as a gift to their enemies (the Trojans) to overtake the city of Troy. In reality, the Greek solders were hidden inside the huge wooden horse, which was hollow, and revealed their presence after the Trojans dragged the "gift" inside Troy and took their enemies by surprise (remember the movie Troy?).

Trojans are executable programs that appear to be useful programs, but in reality have malicious objectives, such as the compromise and damaging of your computer. Trojan horses do not replicate like viruses; instead, they are attached to electronic files you may find useful, such as screensavers, music files, applications, and so forth, to deceive you about their true purpose. After you open the infected file, the Trojan horse activates right under your nose.

Worm

A worm is like a virus that replicates from one computer to another, with the difference that it doesn’t need to travel via a host program. This is accomplished by the worm taking control of features on your computer, such as your email, to send copies of itself to everyone in your address book. When it infects a new computer, it repeats the process, automating its distribution like a chain reaction. By repeatedly doing this, it creates large volumes of network traffic that may slow down Internet communication.