Viruses, worms, and Trojan horses are types of malicious software. A virus attaches itself to a host program and replicates, a worm spreads through network security holes, and a Trojan horse conceals malicious code behind a benign program. Trojan horses can steal passwords and files, launch denial of service attacks, and more. Users can get infected through network holes, downloads, or email attachments. Antivirus software from vendors like Trend Micro, McAfee, and Norton can help protect computers from these threats.