A computer virus is a computer program that can copy itself and infect a computer without a permission of the user. It is not like human viruses. A computer virus can only spread from one computer to other when its host is taken to the uninfected computer, for example, user sending it over a network or the internet or carrying it on a removable medium such as a floppy disk, CD or USB drive. Meanwhile virus can spread to other computers by infecting files on a network file system or a file system that is accessed by another computer. I will talk about computer worm, when your computer has a virus and an anti-virus.
Computer worm is a self-replicating computer program. It uses a network to send copies of itself to other nodes, computer terminals on the network, and it may do so without any user intervention. Unlike a virus, it does not need to attach itself to an existing program. Worms almost always cause harm to the network, if only by consuming bandwidth, whereas viruses almost always corrupt or modify files on a target computer. Many worms have been created which are only designed to spread and don't attempt to alter the systems they pass through. A payload is code designed to do more than spread the worm. It might delete files on a host system or send documents via e-mail. The very common payload for worms is to install a backdoor in the infected computer to allow the creation under control of the worm author. On the other hand, there are worms with good intention for example, The Nachi family of worms tried to download and install patches from Microsoft's website to fix vulnerabilities in the host system.
When your computer has a virus, the point is just like a human virus passed from person to person but the computer virus is passed from computer to computer. The virus can be attached to any file that you copy to your computer. Actually, anytime you download files or connect a removable drives into your computer, you are susceptible to virus. Many viruses are spread through e-mail. Generally, you can not get a virus from simply reading e-mail but the danger is you do not even have to open an e-mail attachment to release the virus. Just opening the e-mail itself can release the dangerous bug. The virus becomes active when you execute a program that contains the virus. For instance, if you download a program from the internet and it was infected with a virus. The virus will attack your computer when you activate the program. Viruses are everywhere. Although, it is rare, viruses have even been found in commercial shrink-wrapped software.
Can we detect it? The easy way is to install anti-virus software into your computer. The anti-virus is a computer software application that has two common methods to detect viruses. The first is using a list of virus signature definitions. This works by examining the content of the computer's memory and the files stored on fixed or removable drives and comparing those files against a database of known virus signatures. The disadvantage of this detection method is that users are only protected from viruses that pre-date their last virus definition update. The second method is use a heuristic algorithm to find viruses based on common behaviors. This method has the ability to detect viruses that anti-virus security firms have yet to create a signature for. The other way to detect virus is making regular backups of data and the Operating Systems on different media, that are either kept unconnected to the system most of the time read-only or not accessible for other reasons. This way will help you if the data is lost through a virus you can use backup to restore your data.
In my opinion, I do not worry about virus too much because I will not download files until I make sure that the files will not have a virus inside. To make sure that my computer will not have any virus, I install anti-virus into it and always scan removable drives, such as USB most virus comes from it. Make sure your friend's USB do not have viruses in it.
