Public health security, the provision of safeguarding health and safety for a designated population during a specific event, is paramount to the success of all mass gatherings (MG). Hajj, the pilgrimage to Makkah, Saudi Arabia - one of the largest annual MG - this year hosted over 2.5 million. Hajj presents an annual public health security challenge of extraordinary dimensions, not only due to its scale but also due to Hajj-specific environmental and host factors. Opportunities for sharing public health security insights from Hajj are limited by the lack of standardized outcome measures. The June 2008 WHO Guidelines on communicable disease alert and response for mass gatherings offer novel opportunities for both Hajj planning and assessment. We discuss the evolution of these Guidelines and the first assessment of their efficacy in a live MG environment. We examine potential opportunities for applying these Guidelines in the novel, intensely dynamic, annually recurrent Hajj environment. We believe Hajj-related findings and outcome measurements using these Guidelines will translate broadly to be of value to many non-Hajj MG sectors. Finally, we suggest areas for outcome assessment and future enquiry and invite the first Hajj-specific communicable disease alert guidelines.