Using an algorithm which could perceive the difference between actual sickness and usage of the common word 'sick', researchers were able to plot and predict when people from a certain area were at risk of picking up a flu bug and the good thing is that twitter can inform you about the risk a week in advance.
The algorithm is based on a model that analyses the spread of infectious diseases among a population, based on twitter messages received and concentration of so-called 'unwell' messages in a certain area.
Source: Medindia