The volume of worms and viruses is increasing, but the rate of successful attacks has dropped, according to a new report from Symantec. The antivirus company’s biannual Internet Security Threat Report found that 4,496 new Windows viruses and worms were released between January and June, up more than 4.5 times from the same period last year. But the daily volume of actual attacks decreased in the first six months of 2004.