Malvertising
Websites or web publishers unknowingly incorporate a corrupted or malicious advertisement into their page. Computers can become infected pre-click and post click. It is a misconception that infection only happens when visitors begin clicking on a malvertisement. "Examples of pre-click malware include being embedded in main scripts of the page or drive-by-downloads. Malware can also auto-run, as in the case of auto redirects, where the user is automatically taken to a different site, which could be malicious. Malware can also be found in the delivery of an ad – where a clean ad that has no malware pre or post click (in its build and design) can still be infected whilst being called. Malicious code can hide undetected and the user has no idea what's coming their way. A post-click malvertisement example: "the user clicks on the ad to visit the advertised site, and instead is directly infected or redirected to a malicious site. These sites trick users into copying viruses or spyware usually disguised as Flash files, which are very popular on the web." Redirection is often built into online advertising, and this spread of malware is often successful because users expect a redirection to happen when clicking on an advertisement. A redirection that is taking place only needs to be co-opted in order to infect a user's computer.
Computer virus
The term "virus" is also commonly, but erroneously, used to refer to other types of malware. "Malware" encompasses computer viruses along with many other forms of malicious software, such as computer "worms", ransomware, spyware, adware, trojan horses, keyloggers, rootkits, bootkits, malicious Browser Helper Object (BHOs), and other malicious software. The majority of active malware threats are actually trojan horse programs or computer worms rather than computer viruses. The term computer virus, coined by Fred Cohen in 1985, is a misnomer. Viruses often perform some type of harmful activity on infected host computers, such as acquisition of hard disk space or central processing unit (CPU) time, accessing private information (e.g., credit card numbers), corrupting data, displaying political or humorous messages on the user's screen, spamming their e-mail contacts, logging their keystrokes, or even rendering the computer useless. However, not all viruses carry a destructive "payload" and attempt to hide themselves—the defining characteristic of viruses is that they are self-replicating computer programs which modify other software without user consent.