A virus is a malicious software that infiltrates the computer of a user without their knowledge. The virus then can execute a variety of malicious operations on the host system.
Fred Cohen originally coined the term in 1983 to describe the way that a computer virus replicates an infectious virus spreading between people.
Unknowingly downloading a file from a website can cause a virus to be installed on computers. The virus attaches to another program so that it can trigger the operation of the virus simultaneously. It can also self-replicate and insert itself into other files or programs.
Each computer virus is unique. Many of them are malicious and can destroy data. Others can remain idle until they are prompted to run.
Viruses may also be spread by viruses that are attached to software and documents. These files can be transferred between computers using a network, USB stick or hard drive or any other data transfer method.
Some viruses use stealth strategies to avoid detection by anti-virus software. This is especially true if the software has an outdated version. Some viruses can infect files without increasing their size. Others can be detected by killing the task in the task manager that runs the antivirus software.