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Which virus type is only executed when a specific condition is met?

Sparse infector


Sparse infector viruses only fire when a specific condition is met.

For example,  maybe the fifth time Calculator is run, whammo--virus execution.


Multipartite viruses attempt to infect both files and the boot sector at the same time.


Metamorphic viruses rewrite themselves each time they infect a new file.


Cavity viruses overwrite unused portions of a file, not change its size.