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.

"We've discovered that certain search terms are good indicators of flu activity," Google said in a statement.

"What this does is it takes Google search terms of influenza-like illness and influenza and it emulates a signal that tells us how much influenza activity there was," Dr. Lyn Finelli, chief of influenza surveillance at the CDC, said in a telephone interview.

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.

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.

Currently, the CDC relies on centers that report on people coming to their doctors with flu-like symptoms, and lab tests that confirm whether a patient has influenza.

But many people with flu never visit a doctor and most doctors treat based on symptoms, rarely giving a flu test.

Either way, the CDC's surveillance data is about two weeks behind.

The Google tool will track flu activity in near real time, the company said.

Early Warningz

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

"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."

Then the CDC can get the word out to hospitals, clinics and doctors offices so they can stock up on flu tests, antiviral drugs and antibiotics for people who get what are known as co-infections -- bacterial infections that worsen a bout of flu.

Two weeks warning also allows people to get vaccinated before flu reaches their community.

Google is not charging for the service. "They are giving it to the world for free," Finelli said.

Google said it would keep individual user data confidential. "Google Flu Trends can never be used to identify individual users because we rely on anonymized, aggregated counts of how often certain search queries occur each week," the company said.

"We rely on millions of search queries issued to Google over time, and the patterns we observe in the data are only meaningful across large populations of Google search users."

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

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.