The number of new viruses has almost tripled in the last six months, an anti-virus vendor said Tuesday, the spike fed by hackers releasing scads of variants to overwhelm defenses.

According to Panda Software, which is headquartered in Spain but has U.S. offices in Glendale, Calif., the count of new viruses has increased 278 percent since the third quarter of 2004.

"Until now, daily updates were considered sufficient for keeping a computer protected from new viruses, and it is exactly this belief that the creators of malicious code are now looking to exploit," said Luis Corrons, the director of Panda's research lab, in a statement. "If they can launch many variants, those that appear after the user has incorporated the new vaccines to their anti-malware solution will have no less than 24 hours to infiltrate the system before the following update."

The trend is a continuation of one seen in early 2004, when the writers of Bagle, MyDoom, and Netsky engaged in a back-and-forth battle, sometimes releasing multiple versions in a single day.

Panda pegged the prolific Mytob worm, which first appeared in February, as the leader in the more-is-better strategy, with 74 variants at last count. Other examples that Panda highlighted were the Kelvir worm (25 version) and Bropia (36). Even older worms are in the action, added Panda: since January 2005, 35 Bagle and 32 MyDoom variants have appeared.

Although Panda, and some other anti-virus firms, roll out new definition files more frequently than daily, Corrons recommended combining "reactive" software such as anti-virus programs with proactive detection technologies capable of detecting malware without having prior knowledge of the threat. Such intrusion detection systems (IDS) typically watch for behaviors that shout "malicious code," rather than compare a potential virus to a signature.