Viruses, worms, and Trojan horses are types of malicious software that can infect computers. A virus attaches itself to files and spreads when an infected file is opened. Worms can spread without human interaction by exploiting network features. Trojan horses appear to be benign files but contain hidden programs that damage systems. All three can slow computers down or access personal information without permission. It is important to use antivirus software, avoid unknown attachments and files, and practice safe internet habits to prevent infection from these types of malware.