Most of the details about worms have already been covered in earlier chapters,
like technical weaknesses and human weaknesses. Worms can also employ the
same techniques that viruses do in order to try and conceal themselves; worms
can use encryption, and can be oligomorphic, polymorphic, or metamorphic.
This chapter therefore only examines the propagation which makes worms
distinct from viruses, beginning with a look at two historically important worms.