A week and a half ago, Panda Security warned about the potential spread of the Conficker worm, a virus spread via USB devices. Since then, Panda has found that nearly six percent of scanned computers were infected, spanning 83 countries.

Originating in China, Conficker worm infection seems concentrated in the United States, Taiwan and Brazil.

PandaLabs found 18,000 infected computers in the US alone. Analyzing two million computers, 5.77 percent, or 115,000 were found to be carrying the Conficker worm.

In the time since the security company issued an "orange alert" regarding the threat level of this malicious program, Panda says variants have been identified that launch brute force attacks to extract passwords from infected computers and internal networks. The company says the frequency of "weak" passwords-common dictionary words, own names-have aided the outbreak.

"Of the two million computers analyzed, around 115,000 were infected with this malware, a phenomenon we haven't seen since the times of the great epidemics of Kournikova or Blaster," says Luis Corrons, Technical Director of PandaLabs.

"This is no doubt an epidemic and the worst may still be to come, as the worm could begin to download more malware onto computers or to spread through other channels. The outbreak of this worm really highlights the need for users to establish strong passwords both on personal computers and corporate networks, as otherwise, an infection could spread across an entire company leaving computers at the mercy of attackers."

Conficker uses a system of social engineering to spread via USB devices. When the Windows options menu appears after inserting a USB device, a prompt appearing to be an option to open a folder to see the files is actually an option to run the program and activate the malware.