DISEASE AND
TRANSMISSIBILITY
Lethal disease is probably an inadvertent and even
unfavorable outcome of infection from the standpoint of a microorganism.
Pathogens that are highly adapted to their host usually spare the majority of
their victims. In many cases, it is to the advantage of the microbe to cause
some degree of illness that may aid its transmission. In other cases, the
interplay between the microbe and the host is subclinical resolution; there may
be damage but no disease. Indeed, many of the most severe infectious diseases
occur when a microorganism adapted to a nonhuman environment finds itself
inadvertently in a human host. The prob-ability of disease is a reflection of
the microbial design to live and multiply within a host balanced against the
host’s capacity to control and limit bacterial proliferation. For certain
microorganisms, such as Streptococcus
pyogenes, contact with susceptible hosts that pos-sess normal host defense
systems renders a certain proportion clinically ill. In contrast, normal
individuals usually shrug off Proteus
and Serratia species. How different
the out-come of this interaction when the host is compromised!
For microbes exclusively adapted to humans,
transmissibility is the key to continued survival. For many organisms, this
entails microbial persistence in the host and in the en-vironment. A stable
pathogen population must retain its viability outside of its preferred niche
and still be capable of infection when it next encounters a susceptible host.
We are still rather ignorant of the microbial factors at play that ensure their
transmissibility from host to host. These conditions are difficult to
recapitulate experimentally. However, the use of bacteria carrying sensitive
reporter molecules will likely permit a better view of transmissibility.
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