The Homeland Security Department’s warning about the virus, known as “Flame,” assured U.S. companies that no infections had been discovered so far inside the U.S. It described Flame as an espionage tool that was sophisticated in design, using encryption and other techniques to help break into computers and move through corporate or private networks. The virus can eavesdrop on data traffic, take screenshots and record audio and keystrokes. The department said the origin is a mystery.