The dramatic decrease is the result of a coordinated attack by security firms and Internet service providers around the globe that took down a network of infected computers known as "the Grum botnet." Grum, one of the world's most prolific spammers, generated around 18 billion emails a day, by FireEye's estimates.

A botnet is a collective of computers infected with malware -- typically without the computer owner's knowledge -- and taken over by an outside attacker. Criminals who gain control of botnets use them for malicious activities like pumping out massive volumes of spam or launching denial-of-service attacks on targeted websites. The bigger the botnet, the more firepower the cybercrimal has at their fingertips.Grum was an especially vast and nasty spammer. First detected in early 2008, its malware infected several hundred thousand computers around the world and churned out huge amounts of pharmaceutical spam advertising cheap drugs.At its peak, Grum was the world's most prolific spam machine, though researchers recently dropped it to the number three spot on their ever-changing list of the world's largest botnets.