Disease X Act This bill establishes a program to develop vaccines, diagnostics, and other medical countermeasures for unknown viral threats. Specifically, the Biomedical Advanced Research and Development Authority must carry out the program in consultation with the Public Health Emergency Medical Countermeasure Enterprise, which coordinates efforts related to medical countermeasures within the Department of Health and Human Services (HHS) and in cooperation with interagency partners. Additionally, the program must prioritize equity and accessibility in the development of the medical countermeasures. The bill also allows HHS to support domestic research, development, and manufacturing of medical countermeasures under the program through grants, contracts, and cooperative agreements.