How can those wanting to infect our PC's with a virus do so when their computers would have it?

Question

Am I missing something here?

Answer 1

Their computers WOULDN'T have it. Viruses are usually spread by either planting it inside another program (Trojan), or by having somebody run it, either by downloading it, or tricking a web browser into running it.

Answer 2

Nearly all viruses (and malware of other types) are spread by the victim running something they shouldn't. This essentially means "don't open or view anything you don't know for certain is benign.

The remainder of viruses not covered by the deployment method above is through "remote exploits". This is nearly always deterred by having your computer behind a router (technically, anything that has NAT).

Having a firewall isn't really necessary, but doesn't hurt to have anyhow.

Answer 3

Computer virus is nothing but a computer program with the ability to modify other programs usually to the determent of the computer system.

Its a dangerous computer program with the characteristic feature of being able to generate copies of itself, and thereby spreading. Additionally most computer viruses have a destructive payload that is activated under certain conditions.

A computer virus is a computer program that can copy itself and infect a computer without permission or knowledge of the user. The original may modify the copies or the copies may modify themselves, as occurs in a metamorphic virus. A virus can only spread from one computer to another when its host is taken to the uninfected computer, for instance by a user sending it over a network or carrying it on a removable medium such as a floppy disk, CD, or USB drive. Additionally, viruses can spread to other computers by infecting files on a network file system or a file system that is accessed by another computer. Viruses are sometimes confused with computer worms and Trojan horses. A worm, however, can spread itself to other computers without needing to be transferred as part of a host. A Trojan horse is a file that appears harmless until executed. In contrast to viruses, Trojan horses do not insert their code into other computer files. Many personal computers are now connected to the Internet and to local-area networks, facilitating their spread. Today's viruses may also take advantage of network services such as the World Wide Web, e-mail, and file sharing systems to spread, blurring the line between viruses and worms. Furthermore, some sources use an alternative terminology in which a virus is any form of self-replicating malware.

The term comes from the term virus in biology. A computer virus reproduces by making (possibly modified) copies of itself in the computer's memory, storage, or over a network. This is similar to the way a biological virus works.

Some viruses are programmed to damage the computer by damaging programs, deleting files, or reformatting the hard disk. Others are not designed to do any damage, but simply replicate themselves and perhaps make their presence known by presenting text, video, or audio messages. Even these benign viruses can create problems for the computer user. They typically take up computer memory used by legitimate programs. As a result, they often cause erratic behavior and can result in system crashes. In addition, many viruses are bug-ridden, and these bugs may lead to system crashes and data loss.