Computer viruses are a major problem for computer users around the world. They are programs that have the ability to reproduce and infect the system. In addition to being able to reproduce, viruses can be designed to perform different destructive actions so that they can destroy programs and data immediately or remain dormant for a certain period and cause damage over a certain period, so knowing viruses can help you keep your system free from viruses. Computer viruses are becoming increasingly sophisticated and are transferred between computer systems using various methods. While email has been the primary method for spreading these recent computer viruses, it is not the only method. A computer virus can also enter a network via CDs, floppy disks, Internet downloads, file-sharing programs, or from remote users connected to a network with an infected system. Once a computer virus enters a network, it can easily spread from one computer to another. After viruses have infected the system, they may wait for a signal before performing damage to the system: time trigger actions and logical actions. Temporal actions are signals triggered by a specific time or date. Logical actions are triggered when specific functions are performed. Most computer viruses have three basic characteristics: a replication process, an activation process, and a target. The replication mechanism looks for other programs to damage, if it finds any it inserts the hidden instruction into the program then changes the program's execution sequence. The trigger mechanism checks for the occurrence of some event. When the event occurs, the computer virus achieves its goal. Computer viruses have affected a significant number of computers in recent years. There is... middle of paper... a firewall to protect the system network, so that viruses that spread via email are prevented from entering the network.4. All emails should be filtered and scanned first so as to avoid surprise attacks, and emails with certain attached files such as .EXE, .COM and .SCR should always be prevented from entering the network or system.5 . All users working in business or at home should be educated and have a clear understanding of how to avoid email viruses sent from unknown sources, so you should be careful when opening attachments.6. Always scan downloads from the Internet; even though the files may seem clean, you should always use percussion so as to prevent viruses from easily entering your system. And this scan should be done somewhere in the network where the virus has no chance to spread through the network.