Patty Kostkova discusses Twitter-based early warning and risk communication of the 2009 swine flu pandemic during a seminar on quantitative methods in social media research held at the OII on 26 September 2012. The need to improve global population monitoring and enhance surveillance of infectious diseases has never been more pressing. Factors such as air travel act as a catalyst in the spread of new and novel viruses. The unprecedented user-generated activity on social networks and online media over the last few years has created real time streams of personal user data which provides an invaluable tool for monitoring and sampling large populations. Epidemic Intelligence relays on the constant monitoring of online media sources for early warning, detection and rapid response; however, the real-time information available in social networks provides a new model of monitoring populations and enhancing the early warning function. The communication of risk in any public health emergencies is a complex task for government and healthcare agencies. This task is made more challenging in the current situation when citizens are confronted with a wide range of online resources, ranging from traditional news outlets to information posted on blogs and social networks. Inevitably, some have greater scientific veracity than others. Twitter is an information source but is also a central hub for the publishing, dissemination, and finding out about online media. In our study, we investigated the role of Twitter during the swine flu pandemics in 2009 from two perspectives. Firstly, we demonstrated the role of the social network to detect an upcoming spike in an epidemic before the official surveillance systems - up to week in the UK and up to 2-3 weeks in the US. Secondly, we illustrated how online resources are propagated through Twitter, and that there is a focus on identifying trusted information sources at the time of the WHO's declaration of the swine flu 'pandemic'. Our findings indicate that Twitter does favour reputable sources but that bogus information can still leak into the network.