Practical Disease Concepts in Epidemiology

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Chapter 3

• Practical Disease Concepts in Epidemiology

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Objectives

• Define disease
• Clarify acute and chronic disease classifications
  according to infectivity and communicability
• Identify various classifications of diseases and
  conditions and their sources and modes of
  transmission
• Understand the major stages in the disease
  process
• Know the five major categories of disease
• Identify the role of zoonosis in communicable
  disease in humans
• Discuss notifiable disease reporting in the
  United States
• Discuss immunity and immunizations against
  infectious diseases
• Identify the change in emphasis of epidemiologic
  study to chronic disease
• Be familiar with common nutritional deficiency
  diseases and disorders
• Be familiar with common chronic diseases and
  conditions

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Define disease

• Disease is an interruption, cessation, or
  disorder of body functions, systems, or organs
• Diseases arise from infectious agents, inherent
  weaknesses, lifestyle, or environmental stresses

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Causes of disease

• Identifying the causes of disease and the
  mechanisms by which they spread remains a primary
  focus in epidemiology
• The science and study of the causes of disease
  and their mode of operation is referred to as
  etiology

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Disease classifications according to infectivity
and communicability

• When an infectious disease is contagious, or
  capable of being communicated or transmitted, it
  is called a communicable disease
• HIV/AIDS
• Cholera
• Influenza

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Transmission of infectious communicable diseases

• Through vertical transmission or horizontal
  transmission
• Vertical transmission refers to transmission from
  an individual to its offspring through sperm,
  placenta, milk, or vaginal
• Horizontal transmission refers to transmission of
  infectious agents from an infected individual to
  a susceptible contemporary

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Pathogens

• Organisms or substances such as bacteria,
  viruses, or parasites that are capable of
  producing diseases
• The ability to get into a susceptible host and
  cause disease is termed invasiveness
• The disease-evoking power of a pathogen is called
  virulence
• Antibiotics work against pathogens because of
  their toxicity

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Classifications of diseases

• Acute disorder with sudden onset, relatively
  severe, and short duration of symptoms
• Chronic less severe but of long and continuous
  duration, lasting over long time periods if not a
  lifetime

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Examples of diseases according to selected
classifications
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Four common stages relevant to most diseases

• Stage of susceptibility
• Stage of pre-symptomatic disease
• Incubation period
• Latency period
• Stage of clinical disease
• Stage of recovery, disability, or death

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A generalized presentation of the natural history
of disease
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Five major categories of disease

• Congenital and hereditary diseases
• Familial tendencies toward certain inborn
  abnormalities
• Injury to the embryo or fetus by environmental
  factors
• Examples Down syndrome, hemophilia, heart
  disease at an early age
• Allergies and inflammatory diseases
• Body reacting to an invasion of or injury by a
  foreign object or substance
• Degenerative diseases
• Deterioration of body systems, tissue, and
  functions
• Metabolic diseases
• Cause the dysfunction, poor function, or
  malfunction of certain organs or physiological
  processes within the body leading to disease
  states.
• Example cells may no longer utilize glucose
  normally causing diabetes
• Cancer
• Characterized by abnormal growth of cells that
  form a variety o tumors, both benign and malignant

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Nine modes of entry into the body of infectious
disease agents

• Respiratory
• Oral
• Reproductive
• Intravenous
• Urinary
• Skin
• Gastrointestinal
• Conjunctival
• Transplacental

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The role of zoonosis in communicable disease in
humans

• Zoonosis any infection or infectious disease
  transmissible from animals to humans.
• The diseases may be endemic or epidemic

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Common zoonotic diseases
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Notifiable disease reporting in the United States

• Notifiable diseases are those of considerable
  public health importance because of their
  seriousness. As a general rule, a disease is
  included on a states list if it
• causes serious morbidity or death
• has the potential to spread, and
• can be controlled with appropriate intervention

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Immunity and immunizations against infectious
diseases

• The immunization process is very important to all
  individuals of the United States
• According to the Centers for Disease Control and
  Prevention, if fewer than 80 of the children in
  a given area have been inoculated for one of the
  contagious diseases, the danger of serious
  outbreaks or localized epidemics remains every
  unvaccinated child is at risk

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Acquired, active, and passive immunity

• Active immunity - the body produces its own
  antibodies. This can occur through a vaccine or
  in response to having a specific disease pathogen
  invade the body
• Passive immunity is acquired through
  transplacental transfer of a mothers immunity
  from diseases to the unborn child

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Diseases for which vaccines are used
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Herd immunity

• Viewed as the resistance a population has to the
  invasion and spread of an infectious disease
• Based on the notion that if a population or group
  is mostly protected from a disease by
  immunizations (say 80 or more) , then the chance
  of a major epidemic occurring is highly limited

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Change in emphasis of epidemiologic study to
chronic disease

• Increasing life expectancy in modern times and
  higher levels of chronic disease has produced a
  change in the emphasis of epidemiologic studies

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Percentage of deaths attributed to selected
infectious and chronic diseases in the
United States
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Malnutrition

• Refers to a condition that arises when the body
  does not get the right amount of vitamins,
  minerals, and other nutrients to maintain healthy
  tissues and proper organ function
• Undernutrition
• Overnutrition

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Malnutrition syndromes
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Common chronic diseases and conditions

• Cardiovascular disease, cancer, chronic
  obstructive pulmonary disease, diabetes mellitus,
  and mental health disorders
• Chronic diseases are not typically caused by an
  infectious agent (pathogen), but result from
  genetic susceptibility, lifestyle, or
  environmental exposures
• Some exceptions are cancers of the cervix, liver,
  and stomach

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Characteristics of chronic diseases

• The latency period for chronic diseases is
  typically more difficult to identify than is the
  incubation period for acute infectious diseases
• This explained by the multifactorial etiology
  which characterizes many chronic diseases

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Cancer risk and aging
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Impairment

• Any loss or abnormality of psychological,
  physiologic, or anatomic structure or function
• Often associated with chronic disease, as it
  represents a decrease in or loss of ability to
  perform various functions, particularly those of
  the musculoskeletal system and the sense organs

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Disability

• Umbrella term for impairments, activity
  limitations, and participation restriction
• Impairments - any loss or abnormality of
  psychological, physiologic, or anatomic structure
  or function
• Activity limitations are difficulties an
  individual may have in executing activities
• Participation restrictions are problems an
  individual may experience in involvement in life
  situations