A large number of biological agents can cause natural or bioterroristic disease outbreaks and each can present in a bewildering number of ways (e.g., a few cases versus many cases, confined to a building versus widely disseminated). This 'problem space' is a challenge for designers of early warning systems for disease outbreaks and the sheer size of this space is a barrier to progress. This paper addresses this problem by deriving nine categories of threats that represent a parsimonious characterization of the problem space. A literature search also identified one or more example outbreaks for each of the nine categories. These outbreaks have occurred in recent times and could be used by researchers in need of actual outbreak data for investigations of the role of different types of surveillance data and algorithms in outbreak detection. The methodological contribution of this research is a Criterion Set of threats for analysis and evaluation of detection systems. This set characterizes the problem space in a tractable manner with less loss of generality than analyses based on one or two selected diseases, which is representative of current analyses.