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‚Äôs 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.