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Hemle wrote these five questions on one concept from the same lesson, the epidemiologic triad, on 27 October 2026. Under each one: the answer, Hemle’s explanation and the sentence on the page it rests on.

1. Question 1 of 5

   Which three elements comprise the traditional epidemiologic triad of disease causation?

   • AReservoir, vector, and vehicle
   • BSymptom, diagnosis, and treatment
   • CAgent, host, and environment
   • DTime, place, and person
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   Answer C: Agent, host, and environment

   As stated on page 52 and confirmed on page 91, the epidemiologic triad consists of an external agent, a susceptible host, and an environment that brings them together. Time, place, and person describe the core components of descriptive epidemiology rather than the causal triad. Ask yourself: What are the three corners of the traditional infectious disease causation model?

   “The triad consists of an external agent, a susceptible host, and an environment that brings the host and agent together.”

   SourcePage 52p.52Page 91p.91

2. Question 2 of 5

   According to the epidemiologic triad model, which statement is true regarding the presence of an infectious agent?

   • AAn agent is rarely necessary for the development of classical infectious disease.
   • BAn agent must generally be present, but its presence alone is not always sufficient to cause disease.
   • CAn agent can produce disease entirely independently of environmental factors and host traits.
   • DAn agent is sufficient by itself to produce clinical disease whenever an exposure occurs.
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   Answer B: An agent must generally be present, but its presence alone is not always sufficient to cause disease.

   Page 52 notes that while an agent generally must be present for disease to occur, its presence alone is not always sufficient to cause disease. Additional factors, such as organism pathogenicity, dose, and host susceptibility, influence whether clinical disease develops. Ask yourself: Does encountering a pathogen always guarantee that a person will become clinically ill?

   “Generally, the agent must be present for disease to occur; however, presence of that agent alone is not always sufficient to cause disease.”

   SourcePage 52p.52

3. Question 3 of 5

   In the context of the epidemiologic triad, how do host factors differ from environmental factors?

   • AHost factors dictate the virulence of the pathogen, whereas environmental factors relate solely to personal hygienic habits.
   • BHost factors are strictly non-modifiable genetic traits, whereas environmental factors refer exclusively to socioeconomic status.
   • CHost factors are intrinsic qualities affecting susceptibility or exposure, whereas environmental factors are extrinsic conditions affecting exposure opportunities.
   • DHost factors encompass extrinsic physical surroundings, whereas environmental factors represent the biologic organism causing illness.
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   Answer C: Host factors are intrinsic qualities affecting susceptibility or exposure, whereas environmental factors are extrinsic conditions affecting exposure opportunities.

   Page 53 describes host factors as intrinsic to the human who can get the disease, such as age, genetics, nutritional status, and behaviors. Environment refers to extrinsic factors that affect the agent and the opportunity for exposure, including climate, geology, and sanitation. Ask yourself: Is the factor an internal characteristic of the individual or an external condition surrounding them?

   “A variety of factors intrinsic to the host, sometimes called risk factors, can influence an individual’s exposure, susceptibility, or response to a causative agent.”

   SourcePage 53p.53

4. Question 4 of 5

   A factory experiences an outbreak of cutaneous anthrax among workers handling imported animal hides without protective clothing in poorly ventilated rooms. Under the epidemiologic triad model, how is the lack of protective antibodies or immune resistance among the workers classified?

   • AAn agent factor
   • BA host factor
   • CAn environmental factor
   • DA vector factor
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   Answer B: A host factor

   Pages 53 and 57 explain that host factors include intrinsic characteristics like immunologic status, age, and susceptibility. The lack of immune resistance or antibodies directly represents an intrinsic host factor. Ask yourself: Does immune resistance reside intrinsically within the human body or externally in the physical environment?

   “Susceptibility and response to an agent are influenced by factors such as genetic composition, nutritional and immunologic status, anatomic structure, presence of disease or medications, and psychological makeup.”

   SourcePage 53p.53Page 57p.57

5. Question 5 of 5

   An investigator states: 'The traditional epidemiologic triad is equally suitable for modeling chronic conditions like cardiovascular disease and cancer as it is for infectious diseases.' What is incorrect about this statement?

   • AThe triad only applies when host susceptibility is 100% universal across the whole population.
   • BThe triad cannot model any illness that involves physical forces or repetitive mechanical injury.
   • CThe triad has proven inadequate for chronic diseases that have multiple contributing causes without a single necessary agent.
   • DThe triad cannot incorporate environmental factors when evaluating chronic diseases.
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   Answer C: The triad has proven inadequate for chronic diseases that have multiple contributing causes without a single necessary agent.

   Page 53 explains that while the triad serves as a useful model for many diseases, it has proven inadequate for cardiovascular disease, cancer, and other conditions that have multiple contributing causes without a single necessary agent. These multifactorial diseases prompted the creation of other models such as causal pies. Ask yourself: Why do multifactorial chronic conditions require models like causal pies instead of the classic triad?

   “While the epidemiologic triad serves as a useful model for many diseases, it has proven inadequate for cardiovascular disease, cancer, and other diseases that appear to have multiple contributing causes without a single necessary one.”

   SourcePage 53p.53

Made in Hemle. The questions, answers, explanations and quoted sentences are word for word, from the quiz as submitted. From Lesson 1 of Principles of Epidemiology in Public Health Practice, a public-domain course by the US Centers for Disease Control and Prevention. The replies, cards and quiz were made by Hemle’s AI; CDC doesn’t endorse Hemle.

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