Stealth viruses escape normal antivirus detection efforts because they contain a unique code. The stealth is a type of polymorphic virus. For example, a boot sector virus may copy the original boot sector to somewhere else on the hard disk, then wait for attempts by other programs to look at the actual boot sector. If the virus detects such an attempt by, say, an antivirus program, it intercepts the attempt and redirects the antivirus program to the original boot sector sitting out on the hard disk. The antivirus program then reports that all is well with the boot sector, and the virus goes undetected.
Some viruses do their dirty work immediately upon execution.
It may be a date, or a time. It may be a certain number of boot ups, or a certain number of times a command is executed. Just like in missiles, the virus payload is whatever damage the virus ultimately delivers. It may be fairly innocuous, like a message appearing on your screen. It may be annoying, like letters falling to the bottom of the screen.