Given the predominance of patients with substance abuse in our review, it is not surprising that prolonged infectious period was the most common outbreak contributing factor.
Table 4 describes the most common outbreak contributing factors, which for 24 outbreaks was prolonged infectiousness.
The source of the outbreak and factors that influenced its spread remain unclear.
We systematically abstracted data on case characteristics, outbreak contributing factors, transmission sites, and interventions (including contact investigation) from reports written by CDC teams upon return from outbreak investigations.
For outbreak contributing factors, hotspots, and interventions, discrete categories were created based on recurring themes in the reports to enable systematic abstraction and quantification of these variables.
We referred to any subsequent presentations and publications describing the investigation (3 – 19 ) to check data quality and accuracy of outbreak contributing factors and transmission sites.
Because multiple outbreak contributing factors and a large number of interventions were documented in CDC reports for each outbreak, we abstracted 2 factors considered by the onsite investigators to be key to fueling the outbreak and 3 interventions not already being used that were either used by the CDC team onsite or recommended to be pivotal to the overall control effort.
The prevalence of self-reported preventive behaviors in response to an anticipated local human-to-human H5N1 transmission outbreak and factors associated with such behaviors have not been examined.
Besides the particular circumstance that provoked the Artibonite's outbreak, other factors may have played a role in the severity of the epidemic in Haiti: the absence of immunity among the population, the higher infectivity of epidemic strains shed in human rice-water stools than of environmental strains, and the role of hypervirulent variant strains in provoking epidemics (24, 29, 30 ).
In the Dutch outbreak, several factors were considered to have contributed to the increase in Q fever cases, including: (i) the high density of farms in the regions where the bacterium is endemic, (ii) asymptomatic infection in the majority of infected animals, and (iii) more efficient diagnostic tests [ 2, 6, 7].
