As previously indicated, only the presence of the infection mechanism is mandatory if the program is to be defined as viral: payload and trigger are optional. Be aware, though, that this is a somewhat simplified model: in some circumstances the dissemination of the viral program itself may be described as the payload. Some worms (and we'll get to defining worms shortly) have been described in this way. Furthermore, if the virus is at all selective about the circumstances under which it will attempt to infect, the infection mechanism may also be said to incorporate a trigger.