Google Flu Trends uses search terms that people put into the Web-based search engine to figure out where influenza is heating up, and notify the U.S. Centers for Disease Control and Prevention in real time. - Mitchell Tsai

Studies indicate that between 35 and 40 percent of all visits to the Internet are begun by people looking for health information. When people are sick, they tend to look up their symptoms. - Mitchell Tsai

Google is keeping the search terms it uses private, but influenza-like illnesses include symptoms such as fever, muscle aches and cough. Sneezing usually occurs with other viruses such as rhinoviruses. - Mitchell Tsai

"One thing we found last year when we validated this model is it tended to predict surveillance data," Finelli said. - Mitchell Tsai

"The data are really, really timely. They were able to tell us on a day-to-day basis the relative direction of flu activity for a given area. They were about a week ahead of us. They could be used ... as early warning signal for flu activity." - Mitchell Tsai

Influenza kills an estimated 36,000 people a year in the United States and 250,000-500,000 globally. - Mitchell Tsai

Experts are keen to track flu activity in case of a pandemic -- a global epidemic of a new and deadly strain of flu that could kill millions within a few months. - Mitchell Tsai