Chapter Overview

Humans are constantly exposed to microbes in their environment, mostly without harm.
When microbes colonize the human body, sometimes they become part of the normal flora and other times, they may cause infection and disease.
Normal flora are bacteria, fungi, and protozoa that reside naturally in the skin, gastrointestinal tract, mucous membranes, and genitourinary tracts, where they provide a stabilizing balance.
Pathogens invade the body and cause harm to the tissues by means of virulence factors.
An infection occurs when an adequate dose of pathogens gains access to the body through a certain route, and subsequently adheres, grows, and disrupts tissues.
Pathogens produce virulence factors such as toxins and enzymes to help them invade the host and damage cells.
Types of infections include local, systemic, blood, latent, primary, secondary, acute, chronic, and asymptomatic.
Infections and diseases often follow predictable patterns involving an incubation period, invasion period, and convalescent period.
The effects of infections and diseases are manifest as symptoms and signs, which may lead to long-term, permanent damage.
Epidemiology is a field that studies the patterns of disease occurrence in a population.
Epidemiologists are concerned with monitoring the numbers of cases; the geographic distribution; the sex, age, and ethnicity of affected people; and the mortality rate.
Disease statistics are reported to various public health agencies to keep track of regional, sporadic, epidemic, and worldwide levels of distribution.
Pathogens originate from humans, animals, food, water, and the environment. Some are transmissible and some are not.
Transmissible infectious diseases may be spread by direct or indirect means by overtly infected people, carriers, vectors, and vehicles.
A significant number of infections are acquired through exposure to the hospital environment.
The causative agents for diseases are determined by a standard set of scientific trials developed by Koch.

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