Tricking users into doing something is social engineering, and this is one
mechanism that worms use to infect machines. Another mechanism that worms
exploit for infection are technical weaknesses. A user doesn't have to be tricked
into running an email attachment, if just viewing the email allows the worm's
code to execute via a buffer overrun. A user doesn't have to be involved at all, if
the worm spreads using buffer overruns between long-running network server
processes on different machines.