A computer worm is a self-replicating computer program. It uses a network to send copies of itself to other nodes (computer terminals on the network) and it may do so without any user intervention. Unlike a virus, it does not need to attach itself to an existing program. Worms always harm the network (if only by consuming bandwidth), whereas viruses always infect or corrupt files on a targeted computer. Worms mainly spread by exploiting vulnerabilities in operating systems, or by tricking users to assist them. Anti-virus and anti-spyware software are helpful, but must be kept up-to-date with new pattern files at least every few days.