Define the following

1. Worm
2. Trojan
3. Bomb

Worm — These viruses are programs which don't affect the functioning of a system. Once these viruses enter, they attack the security zone of a computer system. As a result, they just keep on replicating themselves in the disks.

They also spread through e-mail while using Internet. They use the address book of the system to send themselves through e-mails. These viruses drastically decrease the available memory of the system. For example- Swen, Scalper etc.

Trojan — These computer programs move around the valid programs and get executed with a flashy opening screen. They usually get into the system through e-mail attachments with fascinating names.

Once you open the e-mail, they become active in the background and slowly start deleting/corrupting the files inside the system. They can only spread through user interaction such as opening an e-mail attachment or downloading and running a file from the Internet. For example- Backdoor, Nuker ,etc.

Bomb — These viruses remain inactive for a long time and wait for a specific data or specific event for the disaster. The explosion takes place in the form of a sudden disaster inside the system.

For example, the virus 'Friday 13' waits for the day and date Friday the 13th to delete all the files of the system.