What is the key difference between viruses, worms, and Trojan horses?

A virus needs a host program; a worm is standalone and self-spreading; a Trojan does not self-replicate and relies on tricking the user.

• Viruses - Require a host program to spread; insert themselves into other files; replicate when the infected program runs

• Worms - Standalone software that spreads independently through networks; do NOT require a host program

• Trojan horses - Do NOT reproduce or infect other files; users are tricked into executing them; typically create backdoors

Go deeper:

• What is the difference: viruses, worms, Trojans, and bots? — Cisco — authoritative breakdown adding bots/CnC to the virus-worm-Trojan distinction.