A hybrid virus (sometimes called a multi-part or multipartite virus) is one that combines characteristics of more than one type to infect both program files and system sectors. The virus may attack at either level and proceed to infect the other once it has established itself. Hybrid viruses can be very difficult to eradicate and, unless completely eradicated, will often reinfect the host system repeatedly.
In general, viruses fall into one of three classes: macro viruses, file infectors (also known as program infectors), and system or boot-record infectors. Macro viruses, which are fairly common and often less harmful than other types, infect a word processing application and typically insert unwanted words or phrases. A hybrid virus usually combines the approaches of the two latter types in order to maximize damage and resistance to removal. File infector viruses attack executable files on your hard drive. Each time you run the file, you unknowingly invoke the virus which, in turn, delivers its payload to your system. System or boot-record infectors infect executable code found in certain system areas on a disk, infecting the portion of your hard drive that contains the operating system instructions telling the computer how to start up. These viruses are invoked each time the computer starts.
Because getting rid of a hybrid virus can be such a difficult process, most security experts recommend prevention rather than cure, and suggest that people follow common sense security procedures; these include: running good anti-virus software and keeping virus definitions updated, practicing caution with e-mail and never opening an unexpected attachment or downloading a program from a questionable source.