Google has launched a new website that keep track of flu problem. Google uses some keyword tracking and it’s Google trend technology to show flu trends. According to Google
We have found a close relationship between how many people search for flu-related topics and how many people actually have flu symptoms. Of course, not every person who searches for "flu" is actually sick, but a pattern emerges when all the flu-related search queries from each state and region are added together. We compared our query counts with data from a surveillance system managed by the U.S. Centers for Disease Control and Prevention (CDC) and discovered that some search queries tend to be popular exactly when flu season is happening. By counting how often we see these search queries, we can estimate how much flu is circulating in various regions of the United States.
The same trending technique could be used in tandem with other organizations to track contagious viruses or threats besides the common cold, including AIDS, bird flu, and Africanized honey bees.
The only one limitation to this system is that it does not track the flu trends across the globe.
Source: Google.org Flu Trends
