Botnets threatening the internet

Botnets are computers that had security holes and are under the control of hackers. In this status, they can be used by hackers to send spam messages, take part in attacks on other computers, and similar such negative actions. One tends to think that these are small percentages, but in the latest estimation, it is estimated that upto 25% of all computers are compromised.
Vin Cerf, one of the net luminaries, compared the spread of botnets to an epidemic. Read the article here.
It is very difficult to halt the spread of these botnets unless security is built into operating systms and applications from the beginning.