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The effect that viruses can have is varied by the type of virus you encounter. Most are simply destructive and will destroy files that are important to the proper operation of your computer. Others can open your computer to remote access without your consent or knowledge, record everything you type, and easily steal your passwords or identity. Some viruses will have a payload that will combine many of these symptoms into one.
Email Borne Viruses: The fastest spreading viruses are usually transmitted via email messages. This type of virus usually requires user interaction to infect the computer. Usually just opening an infected email and looking at the message will execute the malicious code. Others will have an attachment accompanied by text that will attempt to use social engineering to entice the computer user to open the message. Once an email-borne virus infects a computer system it will extract contacts from the computer and send emails to everyone from the infected computer and usually imitate the person that is logged onto the computer, thus imitating a familiar person on the recipient's computer.
Macro Viruses: Some viruses will infect Microsoft(TM) Word and Excel files. Some of these will just annoy you and call you names; others have taken down entire corporate email systems for days and cause millions of dollars worth of damage.
Trojan Horse: Some viruses will install a “Trojan horse” program that opens your computer to remote access/control without your consent or knowledge. These types of programs can used by malicious individuals or organizations in order to control thousands of computers to commence attacks on computers networks anywhere in the world. This is called a Distributed Denial of Service (DDoS), Trojan horse programs can also and steal your passwords or identity.
Worms: Worms are malicious programs that are spread via computer networks. This type of program exploits weaknesses/vulnerabilities in Operating Systems or other software(Such as web server software) that is running on another computer. Worms will scan networks looking for other systems that are exploitable. Once an opening is found the remote system will also be infected and participate in the process of spreading the infection. The first widely spread computer worm that made headlines was called Code Red. Code Red exploited a hole in the Microsoft Internet Information Server (ISS).
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