Computer Worm.

A computer worm is a self-replicating malware computer program, which uses a computer network to send copies of itself to other nodes (computers on the network) and it may do so without any user intervention. 

This is due to security shortcomings or deficiencies on the target computer. Unlike a virus, it does not need to attach itself to an existing program. 

Worms almost always cause at least some harm to the network, even if only by consuming bandwidth, whereas viruses almost always corrupt or modify files on a target computer.

Depending on the user activity means reduction in the speed of propagation. A computer worms in the long run reduces connexion in real time of the network, and before that happens is better to kill the node where is connecting the infected computer or terminal.