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"As best we can tell, they took a holiday," said Mr Cox. "The people running Rustock are running a business - albeit an illegitimate one - so maybe they needed time off too."
Rustock's reawakening has also been detected by security firm Symantec Hosted Services.
According to Paul Wood, a senior analyst at the firm, Rustock is now pumping out more than a quarter of all spam circulating worldwide.
On 10 January alone it is expected to have sent out 67 billion junk e-mails, he said.
