One of the most dangerous computer viruses detected in 2013, CryptoLocker ransomware, has affected up to 250,000 computers over the past three months, according to Keith Jarvis, senior security researcher at Dell SecureWorks Counter Threat Unit.

The CryptoLocker is particularly dangerous because there is no antivirus software that can protect computers against it. Most of the victims so far are enterprise users in the UK and the US, but there have also been isolated cases in Australia, Thailand, Canada and India, Jarvis commented in a blog post.

The virus is believed to have been created by a gang located in Russia or Eastern Europe. Initially, business recipients were sent a spam e-mail that was disguised as a computer complaint. When the virus enters the computer it encrypts the entire device, leaving the victims with no option but to follow the sender’s orders and pay the ransom. The money can be sent through an electronic payment system, which makes the gang very difficult to trace.

Jarvis noted that a minimum of 0.4% of victims have paid the ransom, though it is likely that many more have chosen to pay the $300 (£183) to regain access to their hard drives. Still, even at the minimum ransom-paying rate, the gang has obtained hundreds of thousands, possibly even millions of pounds since September.

Even though there is no software that can protect computers against CryptoLocker, there are several measures that can be taken to reduce the risk of becoming a target, Dell SecureWorks Counter Threat Unit said.