Training programmes in intervention epidemiology such as the European Programme for Intervention Epidemiology Training (EPIET) or national field epidemiology training programmes such as the German Programme for Applied Epidemiology (PAE) rely on adapted case studies of past outbreak investigations in order to teach the required methodology to new generations of intervention epidemiologists.

Our aim was to contribute to this growing body of teaching material in a light and entertaining way by publishing a comic book, titled ‘The Disease Detectives’. Our goals were to create an introductory teaching aid to the conceptual framework of outbreak investigations for public health practitioners and to write an entertaining comic book for children aged nine years and older. To our knowledge, comics have not played a large role in teaching medicine or public health, although a comic on facts about sleeping sickness was published in 1997 [1].

Methods
Published case studies and available course material on outbreak investigations were screened, and consensus methods abstracted and rewritten into a fictitious food-borne outbreak of Salmonella after a neighbourhood barbecue. The storyline follows the steps taken in a food-borne outbreak investigation.

Black and white illustrations were hand-drawn on A3 Bristolboard paper with pen and ink and then scanned for digital processing using free open source software (OSS). Scans were first transformed with OSS Inkscape [2] into black and white vector graphics in order to remove grey fringes. After importing and colouring using the OSS GIMP [3] speech bubbles and text were added using Inkscape.

The book and all digital work were put under a Creative Commons 'attribution, non-commercial, share alike' license (4). Creative Commons is a non-profit corporation that helps people who create content define a range of legal controls that allows certain shared use of their material [5].

Results
The Disease Detectives describes how a group of children discover the source of an unknown disease that has made their friends fall ill (Figures 1 and 2). The 14-page comic is freely available in English, German and Norwegian at: http://www.disease-detectives.org
A translation package with all digital art and a short video on how to edit speech bubbles in Inkscape is also available. The comic was presented at the ESCAIDE conference in Stockholm in October 2007.

Discussion
We have created a freely accessible and editable tool to explain the principles of field epidemiology. By wrapping a comic around common methods of outbreak investigations, we have provided a gentle introduction for a non-scientific audience. So far, the comic has already been used in Norway in a health promotion exercise for children (personal communication).

Both the use of OSS and a liberal copyright encourage the free circulation of the comic and its adaptation to different cultural environments. It is very easy to modify the graphics, for instance changing the characters’ skin colours or the text in speech bubbles with readily available, free software. During the ESCAIDE conference, several European public health specialists offered to translate the comic into further languages. To reach our target audience, we will mainly rely on online initiatives such as OER Commons [6], conferences and journal publications.

We will extend the comic with a teaching annex that will use the comic’s panels to describe the steps of an outbreak investigation in greater detail and elaborate statistical and analytical methodology such as relative risks and epidemiological curves.

We hope that The Disease Detectives will kindle readers’ interest in outbreak investigations, serve as an entertaining teaching aid and perhaps even encourage other health professionals to create similar free teaching tools.

Acknowledgements
The authors would like to thank Hege Line Magnussen Løwer for the Norwegian translation and EPIET-C12 for their creative environment.