Last year, the FBI arrested six people they say are responsible for the virus called "DNS Changer".
They were from the country of Estonia, and authorities say they masterminded a problem that affected millions of people around the world.
Monday was the day some experts expected it to hit the U.S. hard, disconnecting tens of thousands of Americans from the internet.
The DNS Changer worked by redirecting web searches on infected computers.
Brad Spiegel from Quality Computer Systems explains how it worked and how you can protect yourself from similar problems.