The number of compromised zombie PCs in botnet networks has quadrupled over the last three months, according to figures from the Shadowserver Foundation. Shadowserver tracks botnet activity and the number of command and control servers. It uses a variety of metrics to slice and dice its figures based in part on the entropy of botnet infections. The clear trend within these figures is upwards, with a rise in botnet numbers of 100,000 to 400,000 (if 30 day entropy is factored into equations) or from 20,000 to 60,0000 (for five day entropy).
Entropy of botnets is calculated on the basis that if no activity is seen from a specific IP for a number of days - either 30, 10 or five - then it is removed from the botnet count. Shadowserver figures suggest the number of command and control servers has actually decreased over the last month, following a spike in activity back in July.