Virus

What a Virus Really Is

A computer virus is malicious software, but the name comes from biology. The term was coined by Fred Cohen in 1985, and the parallel with a biological virus is deliberate: a biological virus invades a living cell, hijacks it, and multiplies. A computer virus does the same to a machine — the only difference is that the victim is a computer system and the virus is a piece of code.

At its core, a virus is a software program created to perform malicious activities and to damage or consume the resources of a computer system. Those resources include CPU time, memory, personal files, and sensitive information. So a virus is not just an annoyance; it actively harms the machine it infects.

How a Virus Spreads

The behaviour mirrors biology closely. Just as a biological virus spreads on contact with another organism, a computer virus spreads when it comes into contact with another system. It does this by copying or inserting its own code into computer programs or software — specifically, into executable files. When an infected file is transferred to another computer, that new machine becomes infected too.

A key point is that a virus does not act immediately. It remains dormant on a system, sitting quietly, until the infected file is opened or executed by a user. Only then is the virus activated. This is why a user's action — running a program or opening a file — is what triggers the damage.

Why Viruses Are Created

Viruses behave differently depending on the reason or motivation behind their creation. The book lists several common intentions:

• Stealing passwords or data — the virus quietly harvests credentials or personal information.
• Corrupting files — the virus damages or destroys data on the system.
• Spamming the user's email contacts — the virus uses the victim's address book to send unwanted mail.
• Taking control of the user's machine — the virus gives the attacker remote command over the system.

So the same label "virus" covers a wide range of behaviours, all united by malice.

Well-Known Viruses

The book names several famous viruses as examples: