The document discusses communicable diseases, emphasizing the multicausation of disease etiology involving interactions among infectious agents, hosts, and environments. It outlines the stages of infection, types of immunity, and the chain of transmission, along with strategies for disease control and prevention, including vaccination and health monitoring. Various definitions and classifications of disease occurrence (e.g., endemic, epidemic, pandemic) are also provided, alongside the importance of immunization schedules and vaccine management.

1. Chapter 25
Communicable Disease
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Principles of Infection and Infectious Disease Occurrence
Biological and epidemiological principles
Multicausation
Spectrum of Infection
Stages of Infection
Spectrum of disease occurrence
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Multicausation
Disease etiology is complex and multicausal.
An infectious agent alone is not sufficient to cause disease; the
agent must be transmitted within a conducive environment to a
susceptible host.
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Epidemiological Triad

2. Host
Agent
Environment
Spectrum of Infection
Not all contact with an infectious agent leads to infection, and
not all infection leads to an infectious disease.
Subclinical infection: no overt symptomatic disease (unapparent
or asymptomatic)
Infections: entry and multiplication of infectious agent in host
Infectious disease and communicable disease:
pathophysiological responses of the host to the infectious agent,
manifesting as an illness (considered a case)
Carriers: people who continue to shed infectious agent without
any symptoms of disease
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Stages of Infection
Latent period
Infectious agent has invaded a host and found conditions
hospitable to replicate
Replication before shedding
Communicable period
Follows latency
Begins with shedding of agent

3. Incubation period
Time from invasion to time when disease symptoms first appear
May overlap with communicable period
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Stages of Infection (Cont.)
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Figure 25-1 From Grimes DE: Infectious diseases, St Louis,
1991, Mosby.
Spectrum of Disease Occurrence
Incidence—new cases in a population
Endemic—diseases that occur at a consistent, expected level in
a geographic area
Outbreak—an unexpected occurrence of an infectious disease in
a limited geographic area during a limited period of time
Epidemic—an unexpected increase of an infectious disease in a
geographic area over an extended period of time
Pandemic—steady occurrence of a disease over a large
geographic area or worldwide
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4. Chain of Transmission
Infectious agents
Reservoirs
Portals of exit and entry
Modes of transmission
Direct
Indirect
Fomites or vectors
Fecal-oral, airborne
Host susceptibility
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Chain of Transmission (Cont.)
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Figure 25-2
Chain of Transmission: Part 1
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Table 25-1
Links
of the Chain

5. Definition
Factors
Infectious agent
An organism (virus, rickettsia, bacteria, fungus, protozoan,
helminth, or prion) capable of producing infection or infectious
disease
Properties of the agent: morphology, chemical composition,
growth requirements, and viability.
Interaction with the host: mode of action, infectivity,
pathogenicity, virulence, toxigenicity, antigenicity, and ability
to adapt to the host
Reservoirs
The environment in which a pathogen lives and multiplies
Humans, animals, arthropods, plants, soil, or any other organic
substance
Portal of exit

6. Means by which an infectious agent is transported from the host
Respiratory secretions, vaginal secretions, semen, saliva, lesion
exudates, blood, and feces
Chain of Transmission: Part 2
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Table 25-1
Links of the Chain
Definition
Factors
Mode of transmission
Method whereby the infectious agent is transmitted from one
host (or reservoir) to another host

7. Direct: person to person
Indirect: implies a vehicle of transmission (biological or
mechanical vector, common vehicles or fomite)
Airborne droplets
Portal of entry
Means by which an infectious agent enters a new host
Respiratory passages, mucous membranes, skin, percutaneous
injection, ingestion, and through the placenta
Host susceptibility
The presence or lack of sufficient resistance to an infectious
agent to avoid or prevent contracting an infection or acquiring
an infectious disease
Biological and personal characteristics (e.g., gender, age,
genetics), general health status, personal behaviors, anatomical
and physiological lines of defense, immunity
Breaking the Chain of Transmission
Controlling the agent
Eradicating the nonhuman reservoir

8. Controlling the human reservoir
Quarantine—during incubation period
Controlling the portals of exit and entry
Isolation of sick persons
Universal precautions
Improving host resistance and
immunity
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Types of Immunity
Natural immunity: an innate resistance to a specific antigen or
toxin
Acquired immunity: derived from actual exposure to specific
infectious agent, toxin, or appropriate vaccine
Active acquired: body produces its own antibodies
Passive acquired: temporary resistance that has been donated to
the host
Primary vaccine failure: failure of vaccine to stimulate any
immune response
Secondary vaccine failure: waning of immunity following an
initial immune response
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Types of Acquired Immunity
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Table 25-2
Type
How Acquired
Length of Resistance
Natural
Active
Natural contact and infection with the antigen
May be temporary or permanent
Passive
Natural contact with antibody transplacentally or through
colostrum and breast milk
Temporary
Artificial

10. Active
Inoculation of antigen
May be temporary or permanent
Passive
Inoculation of antibody or antitoxin
Temporary
Types of Immunity
Herd immunity: a state in which those not immune to an
infectious agent will be protected if a certain proportion
(generally considered to be 80%) of the population has been
vaccinated or is otherwise immune
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Figure 25-3

11. Public Health Control of Infectious Diseases
Control
The reduction of incidence (new cases) or prevalence (existing
cases) of a given disease to a locally acceptable level as a result
of deliberate efforts
Elimination
Controlling a disease within a specified geographic area and
reducing the prevalence and incidence to near zero
The result of deliberate efforts, but continued intervention
measures are required
Eradication
Reducing the worldwide incidence of a disease to zero as a
function of deliberate efforts (e.g., smallpox in 1977)
No need for further control measures
Only possible under certain conditions
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Criteria for Disease Eradication
Human host only; no host in nature
Easy diagnosis; obvious clinical manifestations
Limited duration and intensity of infection
Natural lifelong immunity after infection
Highly seasonal transmission
Availability of vaccine, curative treatment, or both
Substantial global morbidity and mortality rates
Cost effectiveness of campaign and eradication
Integration of eradication with additional public health
variables
Eradication imperative over control measures

12. – CDC (1993)
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Notifiable Infectious Diseases in the United States
Reported weekly in the MMWR
Go to CDC website for latest listing of diseases:
http://www.cdc.gov
Note: State health departments have the responsibility for
monitoring and controlling communicable diseases within their
respective states; they determine which diseases will be
reported within their jurisdiction. Those lists might be longer
than the CDC’s list.
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Vaccines and Infectious Disease Prevention
Immunization is a broad term used to describe a process by
which active or passive immunity to an infectious disease is
induced or amplified.
Immunizing agents can include vaccines as well as immune
globulins or antitoxins.
Vaccination is a narrower term referring to the administration of
a vaccine or toxoid to confer active immunity by stimulating the
body to produce its own antibodies.

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Recommended Immunization Schedules
Recommendations for international immunization practices
determined by WHO
In the United States, AAP and ACIP
Current U.S. recommendations found on CDC website:
http://www.cdc.gov/vaccines
Schedules, footnotes, and educational fact sheets provide
guidelines for practice
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Vaccines: Words of Caution
Information and recommendations on immunizations and
vaccine usage change regularly
Vaccine Information Statements (VISs) that explain the benefits
and risks must be given out before vaccine is administered—a
federal law!! (http://www.cdc.gov/vaccines/hcp/vis/index.html)
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14. Available Vaccines by Type
Live attenuated vaccines (See Textbook Table 25-3)
Viral: measles, mumps, rubella, oral polio, vaccinia, yellow
fever, varicella
Bacterial: BCG (Bacille Calmette-Guérin)
Recombinant: oral typhoid
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Available Vaccines by Type (Cont.)
Inactivated vaccines (See Textbook Table 25-3)
Viral: influenza, polio, rabies, and hepatitis A
Bacterial: typhoid, cholera, and plague
Subunit (fractional): influenza, acellular pertussis, typhoid Vi
and Lyme disease
Toxoid: diphtheria and tetanus
Recombinant: hepatitis B
Conjugate polysaccharide: Haemophilus influenzae type B and
pneumococcal 7-valent
Pure polysaccharide: Pneumococcal 23-valent, meningococcal,
and Haemophilus influenzae type b
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Recommended Vaccine Schedules (Textbook Box 25-6)

15. Children/adolescents
http://www.cdc.gov/vaccines/schedules/hcp/child-
adolescent.html
Adults
http://www.cdc.gov/vaccines/schedules/hcp/adult.html
Travelers
http://wwwnc.cdc.gov/travel/destinations/list
Pregnant women
www.cdc.gov/vaccines/pubs/preg-guide.htm
Health care workers
www.cdc.gov/vaccines/spec-grps/hcw.htm
Specific health conditions
www.cdc.gov/vaccines/spec-grps/conditions.htm
Other special groups
www.cdc.gov/vaccines/spec-grps/default.htm
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Vaccine Storage, Transport,
and Handling
Cold chain
Routes of administration, dosage, and sites
Proper timing and spacing
Hypersensitivity and contraindications
Documentation
Vaccine safety and reporting of adverse events and vaccine-
related injuries (VAERS)
Vaccine needs for special groups
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16. Prevention of Communicable Diseases
Primary prevention
To prevent transmission of an infectious agent and to prevent
pathology in the person exposed to an infection
Secondary prevention
Activities to detect early and effectively treat persons who are
infected
Tertiary prevention
Caring for persons with an infectious disease to ensure that they
are cured or that their quality of life is maintained
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