The number of computers in Japan infected by viruses hit a record 63,657 in 2004, according to a preliminary report Monday from Trend Micro Inc.

The number wipes out the previous record of 47,607 set just last year. The antivirus software maker compiles the report annually based on user inquiries received by its Japanese arm. Monday's report covers the period from Jan. 1 to Dec. 15.

Many of the infections were caused by worms. The worst offender was the Netsky worm, which was responsible for 5,527 cases, if variants derived from the original are included.

A worm called Agent finished second with 2,558 cases, with Bytever coming in third at 2,336, according to the report.

Netsky spreads by e-mail as an attachment, whereas Agent infiltrates computers that are online, destroying and taking data at the same time.

Bytever resides in certain Web sites and infects computers when users browse them.

The report attributed the surge in infections to growing competition among virus writers.

Some virus writers have offered original viruses on the Internet to would-be writers of their variants, making it possible for people to download the original, it warned.

Trend Micro said it will release a report in January covering all of 2004.