Viruses are programs that can copy themselves and infect computers without permission. They spread by downloading infected files from the internet or receiving emails with attachments containing viruses. Viruses avoid detection through encryption that hides their identity. Examples of viruses include Trojan horses, which harm computers quietly by installing malware alongside downloaded programs, and Salami attacks, which steal small amounts of money from accounts repeatedly without notice. To prevent viruses, users should have updated antivirus software and scan their computer weekly.