Unlike a virus-type program, a bug is almost always the result of simple programmer error. This might cause a program not to work at all, generate spurious answers, or conflict with other installed programs. Possible virus effects Some viruses have visible symptoms, such as pop-up windows or altered displays, but some do not. In fact, some seem to exist only to spread themselves to other computers. Microsoft Office macro viruses will sometimes make your Office programs behave erratically, such as not allowing you to save a document except as a template.
Worms can have a severe impact on computer networks.
The major problem the Melissa and Loveletter viruses created was the volume of spurious e-mail they produced. They overloaded networks, which then couldn't handle the delivery of regular mail. Long-term consequences of viruses can vary. Many viruses leave no residual damage at all. However, there are also quite a few that can create serious problems.