South Korean e-security officials estimate over 43,000 web servers in the country have been hit by Code Red II.
The Korean Information Security Agency has noted a steady rise in infections - up from an estimated 13,000 on Wednesday.
One variant of the Internet worm allows hackers to gain easy remote access to infected computers.
According to the Korea Times, government officials have met with ISP and antivirus executives to co-ordinate a national response.
READ MORE
Their new estimate, which is based on the findings of international body the Forum of Incident Response and Security Teams, equates to about 15,000 companies hit by the Code Red variant.
AP