Viruses may contain logic bombs, some benign but mostly malign. A worm is a complete program that reproduces itself. There is an increasing trend to use the generic term 'virus' to cover both viruses and worms. In this section, the generic term 'virus' is used to cover both 'viruses' and 'worms'. There is a clear upward trend in the number and sophistication of attacks using viruses.
Email viruses are prevalent, with the virus normally activating when the attachment is opened.
One prevalent type of virus which relies on the lack of authentication in email is where the email purports to come from an acquaintance, and the virus does a lookup on the infected system for further email addresses. In some cases malicious code has been executed by a user viewing the email, as opposed to traditional infections that have occurred by users selecting the attachment or opening a document. These types of infections occur due to some interoperability and automated settings within some email products. Antivirus products continue to play a game of catch-up, with viruses sometimes able to spread widely before antivirus companies deliver detection strings and antidotes. Web documents often have server-supplied code associated with them which executes inside the web browser.