Sadilek and his team analysed 4.4 million tweets tagged with GPS location data from over 630,000 users in the New York City area over one month in 2010. They trained a machine-learning algorithm to tell the difference between tweets by healthy people – who might say something like “I am so sick of this traffic!” – and someone who is actually sick and showing signs of the flu. The video shows a heat map of flu occurrence over the course of one day, based on their findings.

http://gizmodo.com/5929196/how-twitter-can-predict-when-individuals-will-get-sick