A computer worm is an independent malware computer program that replicates to spread to other computers. He often uses a computer network to spread, relying on security flaws on the target computer to access it. Worms almost always cause at least some damage to the network, even if they only consume bandwidth, while viruses almost always damage or modify the files on a specific computer.
Many worms are designed only to spread, and do not try to change the systems they go through. However, as the Morris and Mydoom worm showed, even these “no-load” worms can cause a major disruption by increasing network traffic and other unwanted effects.