Law enforcement agencies in Europe and the United States have dismantled a network comprising at least 12,000 in computers that had been taken over by criminals, Europol said on Thursday.
The software used to infect the computers was "very sophisticated" but the network was relatively small compared to others uncovered in the past, Europol said in a statement.
Those behind the network, or "botnet," infected computers with the software and may then have sold to others the right to install further malicious programs, said Paul Gillen, the head of operations at Europol's Cybercrime Centre.
Most of the victims were in the United States, Japan, India and Taiwan, Gillen said.
It was impossible to estimate the damage costs, he said, adding that no arrests have been made.
"That's going to take some time," Gillen said. "We've just taken the botnet down last night."
The FBI, Europol and British and Dutch cybercrime police participated in the operation, cooperating with private companies, including Intel, Kaspersky and Shadowserver.