Microorganisms can be found in every ecosystem and in close association with every type of multicellular organism. They populate the healthy human body by the billions as benign passengers (normal flora) and even as participants in bodily functions, for example, bacteria play a role in the degradation of intestinal contents. The role of microorganisms -bacteria, fungi, protozoa, helminths, and viruses- in the initiation and spread of human diseases. Those relatively few species of microorganisms that are harmful to humans, either by production of toxic compounds or by direct infection, are characterized as pathogens.
Most infectious disease is initiated by colonization (that is, the establishment of proliferating microorganisms on the skin or mucous membranes). The major exceptions are diseases caused by introduction of organisms directly into the bloodstream or internal organs. Microbial colonization may result in:
1) elimination of the microorganism without affecting the host, or
2) infection in which the organisms multiply and cause the host to react by making an immune or other type of response. Infectious disease occurs when the organism causes tissue damage and impairment of body function.
Prokaryotic Pathogens
All prokaryotic organisms are classified as bacteria, whereas eukaryotic organisms include fungi, protozoa, and helminths, as well as humans. Prokaryotic organisms are divided into two major groups: the eubacteria, which include all bacteria of medical importance, and the archaebacteria, a collection of evolutionarily distinct organisms. Cells of prokaryotic and eukaryotic organisms differ in several significant structural features.
A. Typical bacteria
Most bacteria have shapes that can be described as either a rod, sphere, or corkscrew. Prokarytoic cells are smaller than eukaryotic cells. Nearly all bacteria, with the exception of the mycoplasma, have a rigid cell wall surrounding the cell membrane that determines the shape of the organism. The cell wall also determines whether the bacterium is classified as gram-positive or gram-negative. External to the cell wall may be flagella, pili, and/or a capsule. Bacterial cells divide by binary fission. However, many bacteria exchange genetic information carried on plasmids' small, specialized genetic elements capable of self-replication—including the information necessary for establishment of antibiotic-resistance.
B. Atypical bacteria
Atypical bacteria include groups of organisms such as Mycoplasma, Chlamydia, and Rickettsia that, although prokaryotic, lack significant characteristic structural components or metabolic capabilities that separate them from the larger group of typical bacteria.
Fungi
Fungi are nonphotosynthetic, generally saprophytic, eukaryotic organisms. Some fungi are filamentous, and are commonly called molds, whereas others "the yeasts" are unicellular. Fungal reproduction may be asexual, sexual, or both, and all fungi produce spores. Pathogenic fungi can cause diseases, ranging from skin infections (superficial mycoses) to serious, systemic infections (deep mycoses).
Protozoa
Protozoa are single-celled, nonphotosynthetic, eukaryotic organisms that come in various shapes and sizes. Many protozoa are free-living, but others are among the most clinically important parasites of humans. Members of this group infect all major tissues and organs of the body. They can be intracellular parasites, or extracellular parasites in the blood, urogenital region, or intestine. Transmission is generally by ingestion of an infective stage of the parasite or by insect bite.
Helminths
Helminths are groups of worms that live as parasites. They are multicellular, eukaryotic organisms with complex body organization. They are divided into three main groups: tapeworms (cestodes), flukes (trematodes), and roundworms (nematodes). Helminths are parasitic, receiving nutrients by ingesting or absorbing digestive contents, or ingesting or absorbing body fluids or tissues. Almost any organ in the body can be parasitized.
Viruses
Viruses are obligate intracellular parasites that do not have a cellular structure. Rather, a virus consists of molecule(s) of DNA (DNA virus) or RNA (RNA virus), but not both, surrounded by a protein coat. A virus may also have an envelope derived from the plasma membrane of the host cell from which the virus is released. Viruses contain the genetic information necessary for directing their own replication, but require the host's cellular structures and enzymatic machinery to complete the process of their own reproduction. The fate of the host cell following viral infection ranges from rapid lysis releasing many progeny virions, to gradual, prolonged release of viral particles.