The cartoon character Pogo, invented by Walt Kelly, once announced to his companions that "the enemy is us." I believe that many of what we refer to as emerging diseases are characterized better as "diseases of human progress." Thus, many major public health crises of the past 2 decades have been infectious in origin. Many, like the outbreaks of Lyme disease and murine typhus, are a natural consequence of human meddling. Similarly, the appearance of infections, like Legionnaires' disease, can be traced to more subtle differences in human behavior and social conventions that have an effect on the microbial world. Thus, the aerosolization of water, now so prominent in the Western world from the widespread use of showers instead of baths to the spraying of produce in large markets to air conditioning, likely has played an important role in the emergence of Legionnaires' disease and also of Mycobacterium avium infection in both healthy and immunocompromised persons.

Legionella pneumophila, the Legionnaires' bacillus, is found in nature as an infectious agent of predatory protozoa. Introduction of this organism, often as part of an aerosol of potable water into the alveolus of the lung, results in the microorganism's finding a new niche in the macrophage instead of in its usual host Acanthamoeba or Hartmanella. More absorbent tampons helped select for a new disease, toxic shock syndrome.

While pathogenic traits of the disease-causing microbes are of consequence, humans and their technology and social behavior have played a major role in providing pathogenic microbes with new venues for their wares. Food poisoning by Escherichia coli O157, Campylobacter, and Salmonella emerged more from food technology and food distribution networks than from any fundamental change in the virulence properties of the bacteria. In a sense, we have provided these bacteria with a moveable feast.

Medicine views pathogens as microorganisms capable of causing disease. The emphasis is on disease, not the microorganism. However, from the microbial standpoint, being pathogenic is a strategy for survival and simply one more remarkable example of the extraordinary diversity of the microbial world. Humans are a home to a myriad of other living creatures. From mouth to anus, from head to toes, every millimeter of our cells exposed to the outside world is inhabited by a rich biology. From the mites that may inhabit the eyebrows to the seething cauldron of more than 600 species of bacteria that inhabit the large bowel, we are a veritable garden of microorganisms. Most of these microorganisms are not only innocuous but play a useful, yet unseen, role in our lives. They protect against the few harmful microorganisms that we encounter each day; they provide vitamins and nutrients and help digest food. We have harbored them so long in our evolution that they are even a necessary part of the developmental pathways required for the maturation of intestinal mucosa and the immune system.

Most microbes are commensal; that is, they "eat from the same table." Others are either commensal or transient microbes that are opportunistic; they can cause disease if one (or more) usual defense mechanism, evolved to restrict microorganisms from normally sterile inner organs and tissue, is breached by accident, by intent (as in surgery and, increasingly, in gunshot wounds), or by an underlying metabolic or even infectious disorder. Nevertheless, a small group of microorganisms often causes infection and overt disease in seemingly healthy persons.

Many of the microorganisms, for example, the typhoid bacillus, gonococcus, tubercle bacillus, and treponema of syphilis, are adapted exclusively to humans; others, for example, Salmonella Typhimurium, can regularly cause disease in humans, animals, birds, and reptiles. The distinct difference between commensal, opportunistic, and pathogenic microbes is that pathogenic microbes have evolved the genetic ability to breach cellular and anatomic barriers that ordinarily restrict other microorganisms. Thus, pathogens can inherently cause damage to cells to forcefully gain access to a new, unique niche that provides them with less competition from other microorganisms, as well as with a ready new source of nutrients.

For microorganisms that inhabit mammals as an essential component of their survival tactic, success can be measured by their capacity to multiply sufficiently to be maintained or be transmitted to a new susceptible host. This is true for commensal and pathogenic organisms alike. However, if the pathogen gains a new niche free of competition and rich in nutrients, it also faces a more hostile environment designed by evolution to restrict microbial entry and, indeed, to destroy any intruders that enter these protected regions. Thus, pathogens have not only acquired the capacity to breach cellular barriers but also, by necessity, have learned to circumvent, exploit, and subvert our normal cellular mechanisms for their own selfish need to multiply at our expense.