Botnets are often used by malicious actors to control and leverage spam email distribution. A botnet is a network of private computers that have been infected with malicious software (malware) and can be controlled remotely by an attacker. The attacker can use the botnet to send spam emails, launch distributed denial of service (DDOS) attacks, and perform other malicious activities.
Viruses and worms are types of malware that can infect computers and cause them to behave in undesirable ways, but they are not typically used to control and leverage spam email distribution in the same way that botnets are. DDOS attacks involve overwhelming a website or online service with traffic in an attempt to make it unavailable to users, but they are not typically used to control and leverage spam email distribution.