Computer viruses can spread rapidly by distributing copies of themselves without permission or knowledge of the user. It is important to use antivirus software to check all files received, especially email attachments, as the file types most commonly used to spread viruses are .exe, .com, .scr, .vbs, .cmd, and .bat files. Trojan horses appear to be useful programs but are actually harmful, so users should only download software from trusted sources and check the origin of all programs and files. A salami attack involves many minor security breaches that together result in significant damage, like an employee stealing small amounts from many bank accounts. Viruses have evolved over time to cause more damage and spread more easily through technology.