WORMS
Unlike viruses, they don’t attack the host, being self-contained programs that propagate across networks and computers. Worms are often installed through email attachments, sending a copy of themselves to every contact in the infected computer email list. They are commonly used to overload an email server and achieve a denial-of-service attack.
Stuxnet
Stuxnet is a special worm that has the ability to infect devices without
internet connection. It is romoured to have been created by the CIA, the
U.S. National Security Agency, and Israeli Intelligence. The purpose of
Stuxnet was to attack Iran's nuclear facilities. However, it has been
mutated many times and spread to other industrial and energy-producing
facilities.
MyDoom
The MyDoom worm was created in 2004 and spread through emails
with subject lines such as "Test", "Error", and "Mail Delivery System".
It became well known for its attempts at trying to hit major companies
such as Microsoft and Google. MyDoom caused millions of dollars in damage.