The company has detected 7,944 new viruses in the first six months of this year, up 59 percent compared to the first half of 2004.
Meanwhile, the average time to infection has decreased. An unprotected and unpatched Windows PC has a 50 percent chance of becoming infected by an internet worm within just 12 minutes of being online, researchers said.
The Zafi-D worm accounted for more than 25 percent of all malware reported to Sophos so far this year. The worm masquerades as a Christmas greeting to trick users into opening its malicious attachment.
Researchers said they have seen a threefold increase in the number of keylogging Trojans this year. The Trojans are used in targeted attacks against organizations in order to steal information, they said.
Earlier this week, SC Magazine reported on a virus that leaked confidential reports from Japanese nuclear power plants.