Chapter 14 Principles of Disease and Epidemiology I. Pathology, Infection, and Disease A. Pathology the scientific study of disease B. Etiology the cause of a disease C. Pathogenesis the manner in which a disease develops D. Infection the invasion or colonization of the body by a pathogenic organism E. Disease when an infection results in change in the state of health II. III. Normal Microbiota A. Normal Microbiota microorganisms that establish a more or less permanent residence but that do not produce disease under normal conditions. B. Transient Microbiota microorganisms that establish for a limited period of time without causing disease C. All organisms have a natural level of microbial growth present in them through out their life, and this community is essential to the proper functioning of the host organism D. The normal microbiota changes over time is the product of a series of complex interactions between the host physiology and the physiologies of the microbiotic communities. E. The normal microbiota can benefit the host by preventing the overgrowth of harmful organisms (microbial antagonism) F. The relationships between the normal microbiota and the host are called symbiosis. There are three types of symbiosis. 1. Commensalism one part of the partnership derives a benefit, but the other is neither harmed nor derives a benefit from the relationship (i.e. surface dwelling bacteria of the eyes and genitals) 2. Mutualism both partners derive a benefit (i.e. E. coli in the gut) 3. Parasitism one partner is harmed while a the other benefits (i.e. most pathogens) G. Opportunistic Pathogens organisms that do not normally cause disease unless the host is compromised or the organism enters a non-normal habitat in the body. The Etiology of Infectious Disease A. The etiology of diseases can be easily determined in some cases (i.e tuberculosis), and remains elusive in others (i.e. Alzheimer s) B. Koch s Postulates the experimental requirements to prove the etiology of a disease developed by Robert Koch. 1. The same pathogen must be present in every case of the disease 2. The pathogen must be isolated from the diseased host and grown in pure culture.
3. The pathogen from the pure culture must cause the disease when introduced into a disease-free host 4. The pathogen must be isolated from the diseased animal and shown to be the original host. C. A number of diseases cannot formally meet the qualifications of Koch s postulates for a variety of reasons, either scientific or ethical. IV. Classifying Infectious Diseases A. Definitions 1. Symptoms changes in body function (i.e. pain) that are subjective 2. Signs objective changes that can be observed and measured (i.e. fever) 3. Syndrome a specific group of symptoms or signs that always accompany a particular disease. 4. Communicable Disease any disease that spreads from one host to another, either directly or indirectly 5. Contagious Disease diseases that are easily spread from one person to another. 6. Noncommunicable Disease diseases that are not spread from person to person. B. Occurrence of a Disease 1. Incidence the number of people in a population who develop a specific disease within a defined time period (i.e. 1000 cases/year), a useful indicator of the spread of a disease. 2. Prevalence the number of people who develop a disease at a specified time (i.e. while the incidence of HPV is estimated at 10,000 cases/year, its prevalence is estimated to be around 250,000 cases), this indicates how severely and long a disease affects a population. 3. Sporadic Disease a disease that occurs infrequently in a given population (i.e. SARS in the US) 4. Endemic Disease a disease that is constantly present in a population (i.e. the common cold) 5. Epidemic Disease a disease that infects many people in a short time frame. 6. Pandemic Disease an epidemic disease that occurs worldwide. C. Severity or Duration of a Disease 1. Acute Disease a disease with a rapid onset and short duration 2. Chronic Disease a disease that develops slowly and persists for an extended period 3. Subacute Disease a disease that is intermediate between the acute and chronic category. 4. Latent Disease a disease whose causative agent remains inactive for an extended period of time before becoming active and producing symptoms.
5. The spread of a disease in a population is governed by the number of susceptible people in a population. A large number of immune people are present in a population is referred to as herd immunity. D. Extent of Host Involvement 1. Local infection the extent of an infection is contained in a small area of the body. 2. Systemic infection the infections are present throughout the entire host. 3. Sepsis an inflammatory condition that arises from the spread of microorganisms and toxins from a local infection. 4. Septicemia a systemic infection arising from multiplication of pathogens or products in the blood. a. Bacteremia septicemia specific to the presence of bacteria b. Toxemia septicemia specific to the presence of toxins c. Viremia septicemia specific to the presence of viruses 5. Primary Infection is an acute infection that causes the initial illness 6. Secondary Infection caused by a opportunistic pathogen after an acute infection weakens the host. 7. Subclinical Infection an infection that does not cause noticeable disease. V. Patterns of Disease A. Predisposing Factors a trait that makes an individual more susceptible to a disease. 1. Gender 2. Poor nutrition 3. Fatigue 4. Age B. Development of Disease 1. Incubation Period the time period between infection and before any signs or symptoms 2. Prodromal Period a period after the incubation in some diseases that is exemplified by mild signs or symptoms. 3. Period of Illness the most severe stage of a disease with overt symptoms. 4. Period of Decline the signs and symptoms subside 5. Period of Convalescence rehabilitation period to regain predisease state. VI. The Spread of Infection A. Reservoirs of Infection continual source of pathogens 1. Human Reservoir a. Humans are by far the biggest source of human pathogens
b. Carrier a human that is infected with, and distributes, a pathogen, but never develops the disease. 2. Animal Reservoir Diseases that occur primarily in animals or livestock and can be transmitted to humans are referred to as zoonoses. 3. Nonliving Reservoir such as soil and water. B. Transmission of Disease 1. Contact Transmission the spread of disease through direct contact. a. Direct Contact Transmission transmission of an agent by physical contact b. Indirect Contact Transmission transmission from a reservoir to a susceptible host by means a nonliving object c. Droplet Transmission transmission of microbes by way of mucus droplets, traveling very short distances (<1 m). 2. Vehicle Transmission transmission by a vehicle such as water, air, or food. 3. Vector animals that carry pathogens from one host to another. a. Mechanical transmission passive transport of a pathogen on an insect s feet on other body parts. b. Biological transmission active process involving the physiology of the vector. VII. IIX. Nosocomial (Hospital-Acquired) Infections A. Nosocomial Infection an infection that does not show evidence of having been present at the time of admission into a hospital. B. In the US, about 2,000,000/year acquire nosocomial infections, nearly 20,000/year die (1%). C. Compromised Host one whose resistance to infection is impaired by disease. Emerging Infectious Diseases A. Emerging Infectious Disease a disease that has a new or changing pattern or incidence. B. The major agencies in the US are Centers for Disease Control (CDC) and the World Health Organization (WHO) X. Epidemiology A. Epidemiology is the science of when and where diseases occur and how they are transmitted in a population. B. The science was pioneered by the works of John Snow and Florence Nightingale, who both utilized the then new science of statistics to show the transmission of disease. C. Three Branches of Epidemiology 1. Descriptive Epidemiology collecting all data that describe the occurrence of the disease under study.
2. Analytical Epidemiology analyzing the statistics of a disease to determine its probable cause. 3. Experimental Epidemiology actively testing a hypothesis about a particular disease. D. The Center for Disease Control and Prevention (CDC) 1. A branch of the US Public Health Service that runs the major epidemiological data collection for the US. 2. The CDC issues the publication Morbidity and Mortality Weekly 3. Notifiable Diseases are those which physicians are required by law to notify the government about when they see a case. 4. Morbidity Rate the number of people affected by a disease in a given period of time in relation to the total population. 5. Mortality Rate the number of deaths resulting from a disease in a population in a given period of time in relation to the total population.