The number of desktops and servers infected by the "downadup" worm has skyrocketed to nearly nine million, according to security firm F-Secure.
That is an increase of more than six million since Thursday last week, when F-Secure warned that the worm was affecting corporate networks and spreading rapidly.
The worm, also known as "conficker", is a large family of network that causes various problems, including locking network users out of their accounts.
F-Secure said in a blog that the spread of the worm was "amazing" and that the situation was not getting better.
The firm ascribes the rapid infection rate to the fact that there are several different variants of the worm.
The most common variant F-Secure has been tracking is creating 250 possible domains each day, the firm said.