Where does spam come from? Why?

Unlike direct mail or telemarketing, email spam has very low per-message cost. In many cases spammers have almost no cost because they are sending email through other people's computers that are infected with a worm or virus, which causes others to bear the cost of their messaging. As a result, despite extremely low response rates, spammers can make a profit fairly easily. The more emails a spammer can send, the greater his profit while the cost remains nearly constant.