The most common The most prominent types of computer virus are reliant on users opening emails or files unaware of the potential threat that they may pose. The emails containing the computer virus usually disguise themselves as casual messages from friends, or enticing and false news items. Without prior knowledge of any possible Virus that is circulating at the time, this type of computer virus is impossible to spot. The user will have no idea that the mail that they have just clicked on and opened has infected their PC and, in some cases, the PC virus will have already sent itself out to all of the email addresses contained within the users email contact list.

As previously mentioned, this type of computer virus can often send itself out using all of the users email contacts, who will then, in-turn, open the infected email, allowing the computer virus to perpetuate itself further via their e-mail contacts. However, there are many other types of computer virus that can piggy-back out from an infected computer as part of a genuine message between friends, while they're completely oblivious to them having a computer virus on their PC, and within their emails.

File transfers and downloads are also targeted by the computer virus, invisible to users. Popular internet and intranet messaging clients are often at risk of passing on and distributing a computer virus further.

Disruption caused by a computer virus
A computer virus will often disrupt an entire system or network, though in some cases the computer virus is benign. Even the benign computer virus can cause disruption by clogging network and email access, in its attempt to further its distribution. The more malicious and negative computer virus will seek a way to cripple an individual computer, or even an entire network; this is often achieved by the automatic deleting or corrupting of files, causing programs or critical files to cease functioning. A computer virus is also able to corrupt computer memory and storage, or simply allow a third party to access sensitive files.

The Trojan computer virus
The ability for access to be given to an unauthorized user is usually attributed to a slightly differing computer virus type, commonly known as a Trojan named so after the Trojan Horse of mythology.

These "Trojan" types of computer virus almost always leave a computer entirely intact and working, making it harder to notice that something may be awry. These computer virus programs will simply create or make use of a back-door in the operating system or internet connection. Due to the common range of ports used to compromise desktop computers, rather than only the Trojan author or propagator having access, anyone who 'probes' the afflicted computer can access the system from a remote point, where they may be untraceable.

The Infection and duplication of a computer virus The computer virus accounts for a large portion of computer and system down-time, due to their predominantly unfavorable and malignant nature. Often capable of crippling software and hardware alike, the computer virus is one of the largest security problems within the computer industry. Computer viruses can often create copies of themselves on each computer to secure their long-term incubation. Viruses that perform this kind of coded survival trait will certainly require some sort of anti-virus software to remove all the copies and registry items; while single copies can sometimes be tracked down to a recent file on a computer, it is still recommended that anti-virus software be used to be certain everything is erased