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| Recent studies have provided evolutionary explanations for much of the variation in mortality among human
infectious diseases. One gap in this knowledge concerns respiratory tract pathogens transmitted from person to
person by direct contact or through environmental contamination. The sit-and-wait hypothesis predicts that
virulence should be positively correlated with durability in the external environment because high durability
reduces the dependence of transmission on host mobility. Reviewing the epidemiological and medical literature,
we confirm this prediction for respiratory tract pathogens of humans. Our results clearly distinguish a highvirulence
high-survival group of variola (smallpox) virus, Mycobacterium tuberculosis, Corynebacterium diphtheriae,
Bordetella pertussis, Streptococcus pneumoniae, and influenza virus (where all pathogens have a mean percent mortality
o0.01% and mean survival time>10 days) from a low-virulence low-survival group containing ten other pathogens.
The correlation between virulence and durability explains three to four times of magnitude of difference in
mean percent mortality and mean survival time, using both across-species and phylogenetically controlled
analyses. Our findings bear on several areas of active research and public health policy : (1) many pathogens used
in the biological control of insects are potential sit-and-wait pathogens as they combine three attributes that are
advantageous for pest control : high virulence, long durability after application, and host specificity ; (2) emerging
pathogens such as the ‘hospital superbug’ methicillin-resistant Staphylococcus aureus (MRSA) and potential bioweapons
pathogens such as smallpox virus and anthrax that are particularly dangerous can be discerned by
quantifying their durability ; (3) hospital settings and the AIDS pandemic may provide footholds for emerging
sit-and-wait pathogens; and (4) studies on food-borne and insect pathogens point to future research considering
the potential evolutionary trade-offs and genetic linkages between virulence and durability. | |
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