2. Pathogens may be shed in feces, in respiratory droplets, on skin cells, in genital secretions, and in urine.


1. Handwashing is a key control measure in preventing diseases that are spread through direct or indirect contact, as well as those that spread via contaminated food.

2. Direct contact occurs when one person physically touches another. Diseases caused by pathogens that have a low infectious dose can be transmitted through direct contact. Pathogens that cannot survive for extended periods in the environment must generally be transmitted through direct contact.

3. Indirect contact involves transfer of pathogens via fomites.

4. Droplet transmission of respiratory pathogens is considered direct contact because of the close proximity involved.

5. Foodborne pathogens can originate from the animal reservoir or from contamination during food preparation.

6. Waterborne pathogens often originate from sewage contamination.

7. Droplet nuclei, dead skin cells, household dust, and soil may carry airborne respiratory pathogens.

8. Airborne transmission of pathogens is the most difficult to control.

9. Mechanical vectors carry the microbe from one place to another.

10. Biological vectors are a required part of the life cycle of a parasite.

11. Prevention of vector-borne disease relies on mosquito, tick, and insect control.

504 Chapter 20 Epidemiology

Portals of Entry

1. The portal of entry of a pathogen can affect the outcome of disease.

Factors that Influence the Epidemiology of Disease

1. The probability of infection and disease is generally lower if an individual is exposed to small numbers of pathogens.

2. Diseases with a long incubation period can spread extensively before the first cases appear.

3. A disease is unlikely to spread very widely in a population in which 90% of the people are immune to the disease agent.

4. Malnutrition, overcrowding, and fatigue increase the susceptibility of people to infectious diseases.

5. The very young and the elderly are generally more susceptible to infectious agents.

6. Natural immunity can vary with genetic background, but it is difficult to determine the relative importance of genetic, cultural, and environmental factors.

20.2 Epidemiological Studies

Descriptive Studies

1. Descriptive studies attempt to identify potential risk factors that correlate with the development of disease by creating a profile of the persons who became ill.

2. Determining the geographical location may give clues about potential reservoirs, vectors, and geographical boundaries that may affect disease transmission.

3. Determining the time that the illness occurred helps distinguish a common-source epidemic from a propagated epidemic. (Figure 20.5)

Analytical Studies

1. Analytical studies try to determine which risk factors are actually relevant to disease development.

2. A retrospective study compares the activities of cases with controls to determine the cause of the epidemic.

3. A prospective study looks ahead, comparing cohort groups, to determine if the identified risk factors predict a tendency to develop disease.

Experimental Studies

1. Experimental studies are generally used to evaluate the effectiveness of a treatment or intervention in preventing disease; they examine the relationship between risk factors or preventative factors and the development of disease.

20.3 Infectious Disease Surveillance

National Disease Surveillance Network

1. The Centers for Disease Control and Prevention collects data on diseases of public health importance and summarizes their status in the Morbidity and Mortality Weekly Report (MMWR); other activities of the CDC include research, assistance in controlling epidemics, and support for infectious disease laboratories. (Figure 20.8)

2. State public health departments are involved in infection surveillance and control; they examine specimens submitted by physicians, local health departments, hospital laboratories, and others.

Worldwide Disease Surveillance

1. The World Health Organization (WHO) is an international agency devoted to achieving the highest possible level of health for all peoples.

20.4 Trends in Disease

Reduction and Eradication of Disease

1. Smallpox has been eradicated.

2. The World Health Organization hopes to soon eliminate other diseases including dracunculiasis, polio, and measles.

Emerging Diseases

1. Emerging diseases include those that are new or newly recognized and familiar ones that are reemerging after years of decline. (Figure 20.10)

2. Factors that contribute to the emergence and reemergence of diseases include microbial evolution, the breakdown of public health infrastructure, changes in human behavior, advances in technology, population expansion, economic development, mass distribution and importation of food, war, and climate changes.

20.5 Nosocomial Infections

1. A hospital can be seen as a high-density population made up of unusually susceptible people, into which the most virulent and antibiotic-resistant microbial pathogens are continually introduced.

Reservoirs of Infectious Agents in Hospitals

1. The organisms that cause nosocomial infections may originate from other patients, the hospital environment, medical personnel, or the patient's own flora.

Transmission of Infectious Agents in Hospitals

1. Diagnostic and therapeutic procedures during hospitalization can potentially transmit infectious agents to patients.

2. Nosocomial infections most often result from medical devices that breach the first-line barriers of the normal host defense.

3. Health care personnel should routinely wash their hands after touching one patient before going on to the next in order to prevent transmission of disease-causing organisms.

Preventing Nosocomial Infections

1. The most important steps in preventing nosocomial infections are to first recognize their occurrence and then establish policies to prevent both their development and spread.

Review Questions

Bacterial Vaginosis Facts

Bacterial Vaginosis Facts

This fact sheet is designed to provide you with information on Bacterial Vaginosis. Bacterial vaginosis is an abnormal vaginal condition that is characterized by vaginal discharge and results from an overgrowth of atypical bacteria in the vagina.

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