It's relatively easy to get a virus or other malware infection these days. There are no "Ad-police" to make sure that ads are not linking to some kind of virus, spyware, or other unwanted piece of software. Today, most malware gets into the system by tricking the user into clicking on something that they see on the web or in an email; effectively bypassing firewalls.

Malware writers use social engineering and exploit detailed knowledge of security vulnerabilities to gain access to their hosts' computing resources. The vast majority of malware (over 99%) target systems running Microsoft Windows and employ a variety of mechanisms to infect new hosts, often using complex anti-detection/stealth strategies to evade antivirus software. Motives for creating viruses can include seeking profit, desire to send a political message, personal amusement, to demonstrate that a vulnerability exists in software, for sabotage and denial of service, or simply because they wish to explore artificial life and evolutionary algorithms.

Malware, short for malicious software, is software used to disrupt computer operation, gather sensitive information, or gain access to private computer systems. It can appear in the form of code, scripts, active content, and other software.

'Malware' is a general term used to refer to a variety of forms of hostile or intrusive software. Malware includes computer viruses, ransomware, worms, trojan horses, rootkits, keyloggers, dialers, spyware, adware, malicious BHOs, rogue security software, and other malicious programs. The majority of active malware threats are usually worms or trojans rather than viruses.

In law, malware is sometimes known as a 'computer contaminant', as in the legal codes of several U.S. states. Malware is different from defective software, which is a legitimate software but contains harmful bugs that were not corrected before release. Malware is commonly disguised as genuine software, and may come from an official company website in the form of a useful or attractive program which has the harmful malware embedded in it. Software such as anti-virus, anti-malware, and firewalls are relied upon to safeguard against malware attacks which helps in identifying and preventing the further spread of malware in the network.

A computer virus is a type of malware that, when executed, replicates by inserting copies of itself (possibly modified) into other computer programs, data files, or the boot sector of the hard drive. When this replication succeeds, the affected areas are then said to be "infected". Viruses often perform some type of harmful activity on infected hosts, such as stealing hard disk space or CPU time, accessing private information, corrupting data, displaying political or humorous messages on the user's screen, spamming their contacts, or logging their keystrokes. However, not all viruses carry a destructive payload or attempt to hide themselves. The defining characteristic of viruses is that they are self-replicating computer programs which install themselves without the user's consent.

Computer viruses currently cause billions of dollars worth of economic damage each year due to causing systems failure, wasting computer resources, corrupting data, increasing maintenance costs, etc. Unfortunately, no currently existing anti-virus software is able to catch all computer viruses (especially new ones). Computer security researchers are actively searching for new ways to enable antivirus solutions to more effectively detect emerging viruses, before they become widely distributed.