Worm.
Worm viruses are belongs to the class of file infectors. So we may say that worm virus infects files and is activated by running of the infected file. They also considered being a variety of so called companion viruses, but unlike the companion viruses worm doesn't connect it operating to some executable file. Their duplication consists in copying of the virus to the different catalogues and sectors of disks. The copy of the virus has no activity; you may say copies sit steel until someday someone will run the file they were copied to. Hope on the running the file to which virus copied itself is rather small so in some cases worm titles its copies to draw user's attention. For example copies may be titled as install.exe.
Some types of the worm operate in rather unusual way, like recording of the copies to zip-files. The best known viruses of this type are ArjVirus and Winstart. ArjVirus is rather dangerous but non-residential file infector. As ArjVirus penetrates into operating systems it find archive arj add-ins files and records its copies there. This virus is able to find archive arj files at both running and executable catalogues. The moment the necessary file is found virus creates a temporary worm virus with com add-ins and random selected name. It could be anything bnvm.com, ythf.com, fufh.com, qomi.com and so on and so forth. The only demand for the name of fake file is that its name shouldn't be longer or less than four bytes. After that worm virus records itself to that file and adds 'trash' in order to make worm-files distinct by the length in every single taken case of archives' infecting.
The newly created worm-file appends to the files that are already have been archived. Appending is performed by means of the Arj-archiver: virus launches the copy processor and directs it to the worm-file. After that worm-file will be added to archive of already packed files. As user requests one of the archived files processor gets a command to run worm-file, and only after that administration is trespassed to program-requestor. Than worm virus deletes temporary worm-file and starts looking for a new target-file, in case if virus find it algorithm performs again, and in case if virus finds no new target-file it goes back to DOS.
Worm viruses are spreading through computer network and after penetration and infecting they don't change files or disks' sectors. Worm penetrates to computers memory by means of network the computer connected to, determines IPs of the other computers that are connected to the same network and sends its copies there. Worm viruses reduce network carrying capacity and slows server's operating speed. Worm virus is not a polymorphic one, so it is easy to detect it and dispose, one won't need a special skills or software to deal with worm virus. Worm may be detected and disposed with the help of any antivirus program. All you have to do is to launch antivirus testing of operating system, and once antivirus find infected files chose option "heal".