According to the different occasions viruses parasitize, they can be divided into three categories: 1. Boot sector virus. Whether it is a hard disk or a floppy disk, there is a boot sector which records some execution programs that must be used when the computer starts up. The boot sector virus will infect these boot sectors and reside in these sectors, using the opportunity of reading the boot record when the system starts up to keep the virus resident in memory. Usually: the virus program first loads into the computer's memory, then modifies the relevant programs (INT 13H or INT 26H) for reading and writing disks under normal circumstances, making the system read and write disk interrupt vector point to the virus itself. In this way, once a disk read/write request is executed, the system first enters the virus program, allowing the virus to spread to the disk being read/written, and then transfers to the original entry of INT 13H or INT 26H to complete the operation. 2. File virus. File viruses generally parasitize in executable programs or files with specific editing formats (such as Word, Excel, Outlook). When the application program is executed, the virus becomes resident in memory. 3. Hybrid virus. This type of virus not only infects the hard disk boot record but also infects disk files. Its transmission method is similar to the previous two, except that it infects both the disk's boot sector and disk files. Original article from Anhui Data Recovery Center: http://www.cndr.cn/wenda/22945.html