A computer virus basically is a program, which has been designed to destroy or corrupt the typical functions of a computer plus its files. Computer viruses attach and place themselves to a host, most often usually a data file, program file or files in your operating system. After it has done so it can it replicates itself and spread the infections to unaffected files.
In order for a computer virus to spread through an entire or part of a computer system or network, the virus must be in active mode or activated. By a virus being activated, it means that a file that is already infected or contains the virus must be executed or opened for the virus to be active. Once activated, the viruses search for particular programs to infect. This mainly depends on how they have been programmed and what they are designed to do. The viruses spread to all your files and folders by duplicating themselves hence multiplying and increasing in number.
Spread through sending and opening of emails.
Email is certainly one of the most commonly known methods of spreading viruses. It is, however, known that emails, which are just Plain text email, do not usually spread viruses. This is because most e-mail viruses are dispersed through attachments attached to email messages. Another way is through email messages, which contain embedded codes that are executable. For viruses enclosed in email messages to attack your documents and files in your computer, they normally involve your computer in execution of some code.
Sharing of software
Sharing of software from one computer to another can facilitate spreading of viruses since some viruses are known to hide among the files of some software. When such software is shared among users, it then changes itself upon installation to an unaffected computer and ends up infecting files of that computer Through the use of removable mass storage.
Most mass storage devices such as the flash disks and memory cards lack virus infection protection and so any data carried using then is prone to infection since the viruses can hide in such medium and it’s mostly noticeable when you have a flash disk with a userspace yet there is no folder visible.
Downloading software’s, media and documents from un-trusted websites.
Many of the websites offering free items are known to have malicious programs attached to such files and on downloading such files you end up downloading viruses, which later affect your computer and corrupts your files and folders.
Spread during booting and rebooting of your computer.
Some viruses are programmed to activate themselves each time your computer is booted. So when such an action occurs the viruses contained in your hard drives ends up spreading to the entire drive.
The opening of written documents
Common viruses include Macro and are specifically designed for macro applications found on your computers such as Microsoft Word, Outlook and Excel. When a document that is infected by the virus is opened using any one of these programs, it leads to the program itself becoming infected, therefore, allowing the viruses to spread to all other documents opened-up inside those programs During running and installation of software.
While installing software to your system, some viruses which remain hidden and are self-contained modify existing software and as you run such software the programs these viruses are duplicated and spread to files and emails.