According to security firm Panda Security, around 6% of the PCs scanned with its online scanning tool were infected with the Downadup worm. Its been just over a week since the worm appeared and researchers are differing on the number of PCs actually infected. Panda says the 6% represents only those PCs whose owners elected to have them scanned. The actual infection rate may be much greater, even as high as 30%. When researchers last week estimated the number of infected machines at 8.9 million, there was a some skepticism over the large number. What security researchers do agree on though, is that this is the biggest and fastest spreading malware in the last several years.