Figure 1.C. Diverse microorganisms cause human disease. Pathogenic organisms are of four main types-viruses, bacteria, fungi, and parasites, which are mostly protozoans or worms. Some important pathogens in each category are listed along with the diseases they cause. The classifications given are intended as a guide only and are not taxonomically consistent; families are given for the viruses; general groupings often used in medical bacteriology for the bacteria; and higher taxonomic divisions for the fungi and parasites. The terms Gram-negative and Gram-positive refer to the staining properties of the bacteria; Gram-positive bacteria stain purple with the Gram stain, Gram-negative bacteria do not.

Figure 1.B. The diversity of microorganisms that are human pathogens.

(a) Human immunodeficiency virus (H IV), the cause of AIDS.
(b) Influenza virus.
(c) Staphylococcus aureus, a bacterium that colonizes human skin, is the common cause of pimples and boils, and can also cause food poisoning .
(d) Streptococcus pyogenes, the bacterium that is the principal cause of tonsillitis and scarlet fever and can also cause ear infections.
(e) Salmonella enteritidis, the bacterium that commonly causes food poisoning .
(f) Mycobacterium tuberculosis, the bacterium that causes tuberculosis.
(g) A human cell (colored green) containing Listeria monocytogenes (colored yellow), a ba cterium that can contaminate processed food, causing disease (listeriosis) in pregnant women and immunosuppressed ind ividuals.
(h) Pneumocystis carinii, an opportunistic fungus that infects patients with acquired immunod eficiency syndrome (AIDS) and other immunos uppressed individuals. The fungal cells (colored green ) are in lung tissue.
(i) Epidermophyton floccosum, the fungus that causes ringworm.
j) The fungus Candida albicans, a normal inhabitant of the human body that occasionally causes thrush and more severe systemic infections.
(k) Red blood cells and Trypanosoma brucei (colored orange), a protozoan that causes African sleeping sickness.
(I) Schistosoma mansoni, the helminth worm that causes schistosomiasis. The adult intestinal blood fluke forms are shown : the male is thick and bluish, the female thin and white . All the photos are false-colored electron microg raphs, with the exception of (l), which is a light micrograph.

Pathogens are infectious organisms that cause disease

Any organism with the potential to cause disease is known as a pathogen. This definition includes not only microorganisms like the influenza virus or the typhoid bacillus that habitually cause disease if they enter the body, but also ones that can colonize the human body to no ill effect for much of the time but cause illness if the body's defenses are weakened or if the microbe gets into the 'wrong' place. The latter kinds of pathogen are known as opportunistic pathogens.

Pathogens can be divided into four kinds: bacteria, viruses, and fungi, which are each a group of related microorganisms, and internal parasites, a less pre cise term used to embrace a heterogeneous collection of unicellular protozoa and multicellular invertebrates, mainly worms. In this site we consider the functions of the human immune system principally in the context of controlling infections. For some pathogens this necessitates their complete elimination, but for others it is sufficient to limit the size and location of the pathogen population within the human host. Figure 1.B illustrates the variety in shape and form of the four kinds of pathogen. Figure 1.C provides a list of common or well-known infectious diseases and the pathogens that cause them.

Over evolutionary time, the relationship between a pathogen and its human hosts inevitably changes, affecting the severity of the disease produced. Most pathogenic organisms have evolved special adaptations that enable them to invade their hosts, replicate in them, and be transmitted. However, the rapid death of its host is rarely in a microbe's interest, because this destroys both its home and its source of food. Consequently, those organisms with the potential to cause severe and rapidly fatal disease often tend to evolve towards an accommodation with their hosts. In complementary fashion, human populations have evolved a degree of built-in genetic resistance to common diseasecausing organisms, as well as acquiring lifetime immunity to endemic dis eases. Endemic diseases are those, such as measles, chickenpox and malaria, that are ubiquitous in a given population and to which most people are exposed in childhood. Because of the interplay between host and pathogen, the nature and severity of infectious diseases in the human population are always changing.

Influenza is a good example of a common viral disease that, although severe in its symptoms, is usually overcome successfully by the immune system. The fever, aches, and lassitude that accompany infection can be overwhelming, and it is difficult to imagine overcoming foes or predators at the peak of a bout of influenza. Nevertheless, despite the severity of the symptoms, most strains of influenza pose no great danger to healthy people in populations in which influenza is endemic. Warm, well-nourished and otherwise healthy people usually recover in a couple of weeks and take it for granted that their immune system will accomplish this task. Pathogens new to the human population, in contrast, often cause high mortality in those infected-between 60% and 75% in the case of the Ebola virus.