A natural pathogen that escapes from a lab or is deliberately released does not count. It must be a pathogen that did not develop in the wild. Modified forms of wild pathogens count iff the modifications affect its behavior such that it is a greater threat to humans.

(If there's a tradeoff involved, it must have become more of a threat on at least one axis. For example, if smallpox is modified to be more deadly but less transmissible, that qualifies.)

If there are significant doubts as to whether the pathogen originated in a laboratory or in the wild, it does not qualify.

Several different pathogens that each infect fewer than 10,000 people won't resolve this market to YES even if the total is greater than 10,000; it must be a single pathogen. (Though it could be via multiple separate outbreaks.)