Forward it to assist for analysis. They may be unaware they are spreading an infected file. The program code is inserted into an existing file which changes the size of the file by the size of the viral code. Once the code has been inserted into a file, if that file is run, the code will then insert itself into many other files of the same time. A worm is a program that can replicate itself by using the capabilities of an operating system or one or more of its components to recreate itself on other systems.
Worms can spread through a network without users sharing files.