Some viruses even can be catastrophic, damaging files, destroying data and crashing systems. At the very least, viruses expand file size and slow real-time interaction, hindering the performance of your machine. The intent of many virus writers is only to infect systems, not to damage them. However, because viruses often are flawed, even benign viruses can interact with other software or hardware and slow or stop the system. Other viruses are more dangerous.
Viruses come from a variety of sources.
Their authors have learned how to extend their reach by using the macro programming language included with Microsoft Word and including a virus in a Word document. Two new, much more destructive Melissa variants have been discovered and are spreading across the world via e-mail. The virus writers have taken the original Melissa code and changed it to create similar but different viruses. The latest variants, Melissa. U and Melissa.