The computer worm responsible for the biggest attack in years has infected at least 1 out of every 16 PCs worldwide, a security company said Wednesday, and may have managed to compromise as many as nearly 1 in 3.

According to Panda Security, almost 6% of the Windows systems scanned with its antivirus technology were found to be infected with “Downadup,” a worm that began aggressive attacks just over a week ago. Panda was one of the first security firms to ring the alarm over Downadup, when it raised its security threat level Jan. 12 as reports of attacks mounted.

Using data from antivirus scans performed by its consumer-grade security software, and by a free online scanning tool it makes available on its Web site, Panda found 111,379 PCs infected with the worm out of a pool of 2 million machines.