Malicious programs that self-replicate across systems are referred to as viruses or worms. Viruses attach themselves to legitimate programs and require user interaction to spread, whereas worms independently exploit vulnerabilities in networks to replicate and propagate without user intervention. Both types can cause significant damage to computer systems, corrupting data, and consuming resources. The terms are often used interchangeably, but understanding their distinctions is crucial for implementing effective security measures to protect against such threats.