14.1 Pathology, Infection, and Disease

• Pathology is the scientific study of disease.

  • Pathology is first concerned with the cause, or etiology, of disease.

  • Second, it deals with pathogenesis, the manner in which a disease develops.

• Infection is the invasion or colonization of the body by pathogenic microorganisms; disease occurs when an infection results in any change from a state of health.

14.2 Human Microbiome

• Human Microbiome Project began in 2007 to analyze microbial communities called microbiomes that live in and on the human body.

• The microorganisms that establish more or less permanent residence (colonize) but that do not produce disease under normal conditions are members of the body’s normal microbiota.

• Others, called transient microbiota, may be present for several days, weeks, or months and then disappear.

• The relationship between the normal microbiota and the host illustrates symbiosis, a relationship between two organisms in which at least one organism is dependent on the other.

• In the symbiotic relationship called commensalism, one of the organisms benefits, and the other is unaffected.

• Mutualism is a type of symbiosis that benefits both organisms.

• In still another kind of symbiosis, one organism benefits by deriving nutrients at the expense of the other; this relationship is called parasitism.

• Microbes such as E. coli are called opportunistic pathogens.

14.4 Classifying Infectious Diseases

• For example, the patient may experience certain symptoms, or changes in body function, such as pain and malaise (a vague feeling of body discomfort).

• The patient can also exhibit signs, which are objective changes the physician can observe and measure.

• A specific group of symptoms or signs may always accompany a particular disease; such a group is called a syndrome.

• A communicable disease is a disease in which an infected person transmits an infectious agent, either directly or indirectly, to another person who in turn becomes infected.

• Chickenpox and measles are also examples of contagious diseases, that is, diseases that are very communicable and capable of spreading easily and rapidly from one person to another.

• A noncommunicable disease is not spread from one host to another.

• The incidence of a disease is the number of people in a population who develop a disease during a particular time period.

• The prevalence of a disease is the number of people in a population who develop a disease at a specified time, regardless of when it first appeared.

• A disease constantly present in a population is called an endemic disease; an example of such a disease is the common cold.

• If many people in a given area acquire a certain disease in a relatively short period, it is called an epidemic disease; influenza is an example of a disease that often achieves epidemic status.

• An epidemic disease that occurs worldwide is called a pandemic disease.

• An acute disease is one that develops rapidly but lasts only a short time; a good example is influenza.

• A disease that is intermediate between acute and chronic is described as a subacute disease; an example is subacute sclerosing panencephalitis, a rare brain disease characterized by diminished intellectual function and loss of nervous function.

• A latent disease is one in which the causative agent remains inactive for a time but then becomes active to produce symptoms of the disease; an example is shingles, one of the diseases caused by Varicellovirus.

• A primary infection is an acute infection that causes the initial illness.

• A secondary infection is one caused by an opportunistic pathogen after the primary infection has weakened the body’s defenses.

• A subclinical infection, also called inapparent infection, is one that doesn’t cause any noticeable illness.

14.5 Patterns of Disease

• A predisposing factor makes the body more susceptible to a disease and may alter the course of the disease.

• The incubation period is the interval between the initial infection and the first appearance of any signs or symptoms.

• The prodromal period is a relatively short period that follows the period of incubation in some diseases.

• During the period of illness, the disease is most severe.

• During the period of decline, the signs and symptoms sub-side.

• During the period of convalescence, the person regains strength and the body returns to its pre-diseased state.

14.6 The Spread of Infection

• These people, called carriers, are important living reservoirs of infection.

• Diseases that occur primarily in wild and domestic animals and can be transmitted to humans are called zoonoses.

• Contact transmission is the spread of a disease agent by direct contact, indirect contact, or droplet transmission.

• Direct contact transmission, also known as person-to-person transmission, is the direct transmission of an agent by physical contact between its source and a susceptible host; no intermediate object is involved.

• Congenital transmission is the transmission of diseases from mother to fetus or newborn at birth.

• Indirect contact transmission occurs when the agent of disease is transmitted from its reservoir to a susceptible host by means of a nonliving object.

• Droplet transmission is a third type of contact transmission in which microbes are spread in droplet nuclei (mucus droplets) that travel only short distances.

• In waterborne transmission, pathogens are usually spread by water contaminated with untreated or poorly treated sewage.

• In foodborne transmission, pathogens are generally transmitted in foods that are incompletely cooked, poorly refrigerated, or prepared under unsanitary conditions.

  • Foodborne transmission frequently occurs because of cross-contamination, the transfer of pathogens from one food to another.

14.7 Healthcare-Associated Infections (HAIs)

• Healthcare-associated infections (HAIs) are infections patients acquire while receiving treatment for other conditions at a health care facility, such as a nursing home, hospital, same-day surgery center, outpatient clinic, or in-home health care environment.

  • Traditionally these were called nosocomial infections.

• Universal precautions are employed to reduce the transmission of microbes in health care and residential settings.

• Standard precautions are basic, minimum practices designed to prevent transmission of pathogens from one person to another and are applied to every person every time.

• Transmission-based precautions are procedures designed to supplement standard precautions in individuals with known or suspected infections that are highly transmissible or epidemiologically important pathogens.

14.8 Emerging Infectious Diseases

• Emerging infectious diseases (EIDs) are diseases that are new or changing, are showing an increase in incidence in the recent past, or show a potential to increase in the near future.

• Bioterrorism, the use of pathogens or toxins to produce death and disease in humans, animals, or plants as an act of violence and intimidation, is another factor that could affect the occurrence of emerging infectious diseases.

14.9 Epidemiology

• The science that studies when and where diseases occur and how they are transmitted in populations is called epidemiology.

  • Descriptive epidemiology entails collecting all data that describe the occurrence of the disease under study.

  • Analytical epidemiology analyzes a particular disease to determine its probable cause.

  • Experimental epidemiology begins with a hypothesis about a particular disease; experiments to test the hypothesis are then conducted.

• Notifiable infectious diseases are diseases for which physicians are required by law to report cases to the U.S. Public Health Service.

• The Centers for Disease Control and Prevention (CDC), a branch of the U.S. Public Health Service located in Atlanta, Georgia, is a central source of epidemiological information in the United States.