The Host-Parasite (Microbe) Relationship

Abstract

No and yes. The skin, oropharynx, upper airways, gastrointestinal tract, and lower female genital tract are full of bacteria, with the highest concentration being in the colon and in dental plaque. Overall, humans are made up of slightly more bacterial cells than human cells; about 40 versus 30 trillion respectively. Although much less prominent, fungi and viruses are also present. In addition to these endogenous microbes, humans come into contact with numerous others on a daily basis—they are inhaled, ingested with food and drink, and picked up on the skin from the environment. Some of these remain in and on the human body for periods of time, while others slough off or die. In contrast to this, there are certain areas of the body where microbial agents are not expected to occur under normal circumstances. These are called sterile sites and include: major organs and their surrounding fluids and capsules; blood and body fluids other than faeces and saliva (yes, including urine!); bone, bone marrow, and joint fluid; subcutaneous tissue, fat, muscle, and tendons; the lower respiratory tract; and some of the genital tract. Microbes only enter these protected sterile sites through various breaches in physical and immunological defences. Again— no and yes. This is a question that has posed much debate over the centuries and seems to evolve as understanding of both humans and microbes expands. Early understanding of infectious diseases was based on the idea that the microbe was an aggressor and the host a passive victim. Currently there is a better understanding of the relationship between microbe and host, which is more of a dance than a war. In order to express an understanding of the relationships between host humans and microbes, a language is required that describes this confusing and complex interaction, especially considering that knowledge in this field is still evolving. The bacteria that reside in or on human bodies on a semi- permanent basis are called normal flora, or indigenous microbiome. Each person has a relatively unique set of fairly stable microbes likely determined by early experience, and continued exposures and diets.