Which type of malware spreads from one system to another or from one network to another and causes similar types of damage as viruses do to the infected system?

A rootkit allows an attacker to maintain hidden access to a system but does not primarily describe a method of self-replication or widespread damage across systems.

A Trojan disguises itself as legitimate software to trick users but typically does not self-replicate and spread to other systems.

A worm is a standalone malicious program that replicates itself and spreads to other computers or networks without human interaction, often exploiting network vulnerabilities to propagate. It causes damage similar to viruses, such as consuming bandwidth, deleting files, or installing other malicious software.

Adware displays unwanted advertisements and is not primarily designed to self-replicate and spread across systems to cause damage.