before computer networks became widespread, viruses spread on removable media, particularly floppy disks. in days of personal computer, many users regularly exchanged information , programs on floppies. viruses spread infecting programs stored on these disks, while others installed disk boot sector, ensuring run when user booted computer disk, inadvertently. personal computers of era attempt boot first floppy if 1 had been left in drive. until floppy disks fell out of use, successful infection strategy , boot sector viruses common in wild many years. traditional computer viruses emerged in 1980s, driven spread of personal computers , resultant increase in bulletin board system (bbs), modem use, , software sharing. bulletin board–driven software sharing contributed directly spread of trojan horse programs, , viruses written infect popularly traded software. shareware , bootleg software equally common vectors viruses on bbss. viruses can increase chances of spreading other computers infecting files on network file system or file system accessed other computers.
macro viruses have become common since mid-1990s. of these viruses written in scripting languages microsoft programs such microsoft word , microsoft excel , spread throughout microsoft office infecting documents , spreadsheets. since word , excel available mac os, spread macintosh computers. although of these viruses did not have ability send infected email messages, viruses did take advantage of microsoft outlook component object model (com) interface. old versions of microsoft word allow macros replicate additional blank lines. if 2 macro viruses simultaneously infect document, combination of two, if self-replicating, can appear mating of 2 , detected virus unique parents .
a virus may send web address link instant message contacts (e.g., friends , colleagues e-mail addresses) stored on infected machine. if recipient, thinking link friend (a trusted source) follows link website, virus hosted @ site may able infect new computer , continue propagating. viruses spread using cross-site scripting first reported in 2002, , academically demonstrated in 2005. there have been multiple instances of cross-site scripting viruses in wild , exploiting websites such myspace (with samy worm) , yahoo!.
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