A new variant of the PushDo trojan has infected more than 11,000 computers in a single day, according to Bitdefender.
The botnet was found to be most prevalent in Vietnam, India and Indonesia.
Catalin Cosoi, chief security strategist at Bitdefender, told Help Net security: "Yesterday, we managed to successfully intercept PushDo traffic and gain some idea of the size of this botnet.
"The sheer scale of this criminal operation, unsophisticated as it may be, is rather troubling and there are indications that the botnet is still in a growth phase."
Keys used to protect the botnet's communication protocol have altered, according to Bitdefender, though the protocol itself remains the same.
It is also using a new domain generation algorithm based heavily on the previous one, allowing it to avoid detection by regularly shifting the command and control (C&C) server that issues instructions to the virus.
"New PushDo binaries contain now an encrypted overlay, having the role of a check-up," Bitdefender said.
"If the conditions specified in the overlay aren't met, the sample doesn't run properly."
Binary files are those that have been compiled by coders so the software is functional, as opposed to the raw source code.