(Segment 1 of 4) Clade X is a day-long pandemic tabletop exercise hosted by the Johns Hopkins Center for Health Security on May 15, 2018, in Washington, DC. The exercise simulates a series of National Security Council-convened meetings of 10 US government leaders, played by individuals prominent in the fields of national security or epidemic response. Drawing from actual events, Clade X identifies important policy issues and preparedness challenges that could be solved with sufficient political will and attention. These issues were designed in a narrative to engage and educate the participants and the audience. Lessons learned from Clade X will be distilled by the Center and shared broadly. More information is available at https://www.centerforhealthsecurity.org/our-work/exercises/2018_clade_x_exercise/index.html CLADE X IS A FICTIONAL EXERCISE AND DISEASE This training tabletop exercise is based on a fictional scenario. The inputs experts used for modeling the potential impact were fictional. It is a teaching and training resource for public health and government officials.