An interpreted virus is composed of source code that can be executed only by a particular application or service. Interpreted viruses have become very common because they are much easier to write and modify than other types of viruses.
A relatively unskilled attacker can acquire an interpreted virus, review and modify its source code, and distribute it to others. There are often dozens of variants of a single interpreted virus, most with only trivial changes from the original. The two major types of interpreted viruses are