This document discusses viruses and countermeasures against them. It begins by defining viruses and their operation modes and structure. It describes different types of viruses like macro viruses, email viruses, and Trojan horses. It then discusses recent malicious attacks like Code Red and Nimda. The document outlines various virus countermeasures like prevention, detection, and reaction techniques. It describes advanced techniques like digital immune systems, behavioral blocking software, and antivirus software programs. It concludes by emphasizing the importance of installing antivirus applications, regularly scanning for viruses, gaining knowledge about how viruses work, and using basic internet security applications.