The resident or normal flora include bacteria, fungi, and protozoa.

There are two types of cutaneous populations of microbes: the transients, which cling but do not grow on the superficial layers of the skin, and the more permanent resident flora, which reside in the deeper layers of the epidermis and its glands.

Flora of the alimentary canal are confined primarily to the mouth, large intestine, and the rectum.

Flora of the respiratory tract extend from the nasal cavity to the lower pharynx.

Flora of the genitourinary tract are restricted to the urethral opening in males, and to the urethra and vagina in females.

Axenic animals (having no normal flora) are used to study the role of normal flora in animal health, particularly in relationship to the immune response.

The Progress of an Infection
Microbial infections result when a microorganism penetrates host defenses, multiplies, and damages host tissue. The pathogenicity of a microbe refers to its ability to cause infection or disease. The virulence of a pathogen refers to the degree of damage it inflicts on the host tissues.

True pathogens cause infectious disease in healthy hosts, whereas opportunistic pathogens become infectious only when the host immune system is compromised in some way.

The site at which a microorganism first contacts host tissue is called the portal of entry. Most pathogens have one preferred portal of entry, although some have more than one.

The respiratory system is the portal of entry for the greatest number of pathogens.

The infectious dose, or ID, refers to the minimum number of microbial cells required to initiate infection in the host. The ID varies widely among microbial species.

Fimbriae, flagella, hooks, and adhesive capsules are types of adherence factors by which pathogens physically attach to host tissues.

Exoenzymes, toxins, and anti-phagocytic factors are the three main types of virulence factors pathogens utilize to combat host defenses and damage host tissue.

Exotoxins and endotoxins differ in their chemical composition and their tissue specificity.

Anti-phagocytic factors produced by microorganisms include leukocidins, capsules, and factors which resist digestion by white blood cells.

Patterns of infection vary with the pathogen or pathogens involved. They range from local and focal to systemic.

A mixed infection is caused by two or more microorganisms simultaneously.

Infections can be characterized by their sequence as primary or secondary and by their duration as either acute or chronic.

An infectious disease is characterized by both objective signs and subjective symptoms.

Infectious diseases which are asymptomatic or subclinical nevertheless less produce clinical signs.

The portal of exit by which a pathogen leaves its host is usually, but not always, the same as the portal of entry.

The portals of exit and entry determine how pathogens spread in a population.

Some pathogens persist in the body in a latent state; others cause long-term diseases called sequelae.

Epidemiology: The Study of Disease in Populations
Epidemiology is the study of the determinants and distribution of all diseases in populations. The study of infectious disease in populations is just one aspect of this field.

Data on specific, reportable diseases is collected by local, national, and worldwide agencies.

The prevalence of a disease is the percentage of existing cases in a given population. The disease incidence of morbidity rate is the ratio of newly infected to uninfected members of a population.

The disease frequency is described as sporadic, epidemic, pandemic, or endemic.

Causative agents of infectious disease must be isolated and identified according to Koch's postulates.

The primary habitat of a pathogen is called its reservoir. A human reservoir is also called a carrier.

Animals can be either reservoirs or vectors of pathogens. An infected animal is a biological vector. Uninfected animal, especially insects, which transmit pathogens mechanically are called mechanical vectors.

Soil and water are nonliving reservoirs for pathogenic bacteria, protozoa, fungi, and worms.

A communicable disease can be transmitted from an infected host to others, but not all infectious diseases are communicable.

The spread of infectious disease from person to person is called horizontal transmission. The spread of infectious disease from parent to offspring is called vertical transmission.

Infectious diseases are spread by either direct or indirect routes of transmission. Vehicles of indirect transmission include soil, water, food, droplet nuclei, aerosol nuclei, and fomites (inanimate objects).