IT contractors have been warned that the botnet which infected nearly 13 million computers was just a "drop in the ocean", according to an IT security firm.

The Mariposa botnet spread across 190 countries collecting account information from credit card users, social media sites and email services along the way.

Following a collaborative effort which included the Spanish police, the FBI and Panda Security, it has now been shut down and the Spanish bot herders arrested.

However, while Mariposa was a huge individual botnet, it's just a "drop in the ocean" in the fight against cybercrime, according to Luis Corrons, technical director of Panda Security.

Mariposa managed to spread itself effectively through peer-to-peer programs and USB ports, said Mr Corrons.

While firewalls can protect against attacks from the internet, "you cannot avoid having someone working in your company bringing with him a USB stick and plugging it into his computer, infecting it," Mr Corrons added.

IT security firm BitDefender recently reported that the number one malware threat is a trojan that spreads through removable devices.