More than 250,000 people, including many in the UK, may have been affected, after the FBI shut down servers today as part of their fight against computer crime.
In November 2011 the FBI brought charges against an Estonia-based gang who used a malware known as "DNS Changer" to net more than £9m. The virus hijacked web searches, forcing victims to see certain adverts.
The FBI had delayed shutting down the servers until today to allow time for victims to disinfect their computers. Millions of users have since been alerted, but it is feared that around 250,000 unaware victims worldwide will have lost internet access when the FBI shut down the servers at 00.00 EDT (5am GMT) this morning.
The FBI arranged for a private company to run a website, http://www.dcwg.org, which PC users could visit to instantly check if their computer has been infected. By now it's too late for those who have already lost access. They will have to rely on telephone helplines set up by internet service providers.