A worm can exploit security vulnerabilities to spread itself to other computers without needing to be transferred as part of a host

Trojan horse is a program that appears harmless but has a hidden agenda. Worms and Trojans, like viruses, may cause harm to either a computer data, functional performance, or networking throughput, when they are executed.

Some viruses and other malware have symptoms noticeable to the computer user, but many are surreptitious.