The Conficker worm caught the security industry off guard when it first emerged late last year, Symantec has admitted.
The security software giant said that though worms similar to Conficker had been seen in the past, it had been a number of years since worms of any description had dominated the threat landscape.
"Conficker was the big event of the early part of the year. It caught us off guard - we hadn't seen a worm like this for a few years being used to spread malware. There was a time when we saw worms every day, but that was a few years ago," said Orla Cox, senior manager of the Symantec Security Response division.
Conficker infected millions of PCs in the early part of the year though its role was not to attack the computers it infected - merely to enable other malware programs to do their work.
However, a wide-scale attack, which many had predicted would happen on 1 April, never materialised.
"We're yet to see what Conficker is capable of doing in terms of its full potential. It's there to be a 'dropper' for something else such as rogue anti-virus-type software," said Paul Wood, senior analyst at MessageLabs, which was bought by Symantec earlier this year.