Your computer is up and running, but your virus problems aren't over yet. After the initial transfer to the boot record, viruses have a number of opportunities to infect the system. It's interesting to note that some viruses can tell they've already infected a program while others cannot. It's the stupid ones that, after a while, are easy to spot. Polymorphic viruses are anything but stupid.
Polymorphism means the occurrence of different forms, stages or types.
And that's what a polymorphic virus does. It changes itself to avoid detection. Programs possess certain binary characteristics called signatures. These can be anything from the file header to a particular series of instructions. Early viruses were easily detected by scanning for their unique signatures.