Malware, short for malicious software, is any software used to disrupt computer or mobile operations, gather sensitive information, gain access to private computer systems, or display unwanted advertising.

Before the term malware was coined by Yisrael Radai in 1990, malicious software was referred to as computer viruses.

The first category of malware propagation concerns parasitic software fragments that attach themselves to some existing executable content.

The fragment may be machine code that infects some existing application, utility, or system program, or even the code used to boot a computer system.

Malware is defined by its malicious intent, acting against the requirements of the computer user, and does not include software that causes unintentional harm due to some deficiency.

Email filtering is the processing of email to organize it according to specified criteria.

Most often this refers to the automatic processing of incoming messages, but the term also applies to the intervention of human intelligence in addition to anti-spam techniques, and to outgoing emails as well as those being received.

Various anti-spam techniques are used to prevent email spam (unsolicited bulk email).

No technique is a complete solution to the spam problem, and each has trade-offs between incorrectly rejecting legitimate email (false positives) vs.

not rejecting all spam (false negatives) – and the associated costs in time and effort.

Electronic mail, or email, is a method of exchanging digital messages between people using digital devices such as computers and mobile phones.

Email first entered substantial use in the 1960s and by the mid-1970s had taken the form now recognized as email.

Email operates across computer networks, which today is primarily the Internet.

Some early email systems required the author and the recipient to both be online at the same time, in common with instant messaging.

Today's email systems are based on a store-and-forward model.

Email servers accept, forward, deliver, and store messages.

Neither the users nor their computers are required to be online simultaneously; they need to connect only briefly, typically to a mail server or a webmail interface, for as long as it takes to send or receive messages.

A computer virus is a type of malicious software program ("malware") that, when executed, replicates by reproducing itself (copying its own source code) or infecting other computer programs by modifying them.

Infecting computer programs can include as well, data files, or the "boot" sector of the hard drive.

When this replication succeeds, the affected areas are then said to be "infected" with a 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, trojan horses, keyloggers, rootkits, spyware, adware, 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 install themselves without user consent.