Bombs

Trojans

In the second (trigger)stage, the virus monitors the activities of the system and when a predefined event occurs, it triggers off. This trigger can be made to set off at a particular time or an event depending on the physical condition of the disk (e.g. when it is half full), the number of times a program is used, the amount of memory used or almost anything; even on the birthday of the writer, depending on his fancy. Once this trigger goes off, the virus becomes active and starts replicating. At this stage itself, or at an optional final stage, the virus “bombs” (explodes) and performs its last act (very often destructive) in a grand finale.

Basically, viruses infect files or certain areas of the disk, or even both of them. It is clear that if a virus wants to spread via executable programs (EXE or COM files), it must attach some extra code onto that program. The viral code may be added to the file or it may replace some existing code. For such a virus to become active, the infected program necessarily has to be executed.

For viruses which infect the critical areas of the disk, the computer has to be booted from the infected disk. Thus, only if you run an infected file or boot from an infected disk, can viruses get into the system.

Programs which do not have any pre-trigger phase are known as Bombs. These programs do not infect other programs, instead they “explode” on execution without any delay. Damage from such programs is direct and immediate. However, since these programs do not propagate by infecting other programs, chances of a widespread epidemic are relatively slim.

Trojans are programs which claim to be something useful but are intentionally programmed to be malicious. These programs contain some harmful code along with the regular code required for normal execution. The program runs properly except that the destructive code triggers off at a set time or event. For example, a program may claim to be a new version of a reputed program but may be a Trojan, that is, contain harmful code.