The main difference between a virus and a worm in computer security lies in their propagation methods. A virus attaches itself to a legitimate program or file and is activated when the host file is executed, requiring human intervention to spread. In contrast, a worm is a standalone malware that self-replicates and spreads across networks independently, often exploiting vulnerabilities in operating systems or applications. This difference makes worms particularly dangerous, as they can rapidly infect numerous systems without user action, leading to larger-scale security breaches.