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Epidemiology and Public Health
Nester Chapter 20
Notebook, Page 281
Introduction to Epidemiology

Definition
– Epidemiology is the study of the frequency
and distribution of disease

Importance of epidemiology
– Allows development of guidelines for the
prevention and control of certain diseases

Disciplines that make-up epidemiology
– Ecology, Microbiology, Sociology, Statistics
& Psychology …….
Epidemiology

Definitions and Descriptive Terms
– Attack rate
• Proportional number of cases developing in the
population that was exposed to an infectious
agent
– Communicable disease
• An infectious disease that can be transmitted
from person to person
– Endemic
• Disease or other occurrence that is constantly
present in a population
Epidemiology
– Epidemic
• Disease or other occurrence with a higher
incidence than expected
– Herd immunity
• Occurs when a critical conc. of immune hosts
prevents spread of an infectious agent
– Incidence
• # of new cases of a disease in a population at
risk during a specific time period
Epidemiology
– Index case
• First identified case in an outbreak or epidemic
– Morbidity
• Illness; Expressed as rate of illness in a given
population at risk
– Mortality
• Death; Expressed as a rate of death in a given
population at risk
– Non-communicable disease
• A disease not transmitted from one host to
another
Epidemiology
– Outbreak
• A cluster of cases occurring during a brief
period; affecting a specific population; may
herald an epidemic
– Pandemic
• Worldwide epidemic
– Prevalence
• Total number of cases in a given population at
risk at some point in time
Spread of Disease
– Reservoir
• Natural habitat of disease-causing organism
– Mode of transmission
– Portal of entry
• Surface or orifice through which diseaseproducing agent enters the body
– Portal of exit
• Surface or orifice through which diseaseproducing agent exits and disseminates
Spread of Disease

Types of reservoirs
– Humans
• Communicable diseases e.g. smallpox
– Animal reservoirs- zoonotic diseases
• Rodents-plague
• Giardia
• Salmonella
– Environmental reservoirs
• Soil
• Water
Spread of Disease

Portal of exit
– Intestinal organisms
– Respiratory organisms
– Skin organisms
– Genital tract organisms
Spread of Disease

Mode of Transmission
– Person to person transmission
• Direct contact
– Horizontal spread
– Vertical spread
• Droplet transmission
– Droplet nucleii in the inhaled air
• Indirect contact
– Fomite transmission
Spread of Disease

Mode of transmission (continued)
– Contaminated food or water
– Infected cut from an organism in the soil
– Arthropod vectors
• Ticks
• Fleas
• Mosquitos
Spread of Disease

Portal of Entry
– Respiratory tract
– Digestive tract
– Urinary tract
– Genital tract
– Through skin
Factors that influence the
Epidemiology of Disease



The infectious dose
The incubation period
Population characteristics
– Immunity to the pathogen
– General health
– Age
– Gender
– Religious and cultural practices
– Genetic background
Types of Epidemiological Studies

Three major types of studies
– Descriptive studies
– Analytical studies
– Experimental studies
Types of Epidemiological Studies

Descriptive studies occur immediately a
disease outbreak is recognized
– Person
– Place
– Time
• Common source outbreak
• Propagated outbreak
Types of Epidemiological Studies

Analytical studies are designed to
determine which of the potential risk
factors identified in the descriptive
studies are actually relevant in the
spread of disease
– Retrospective studies
– Cross-sectional studies
– Prospective studies
• Cohort groups
Types of Epidemiological Studies

Analytical studies are designed to
determine which of the potential risk
factors identified in the descriptive
studies are actually relevant in the
spread of disease
– Retrospective studies
Types of Epidemiological Studies

Analytical studies are designed to
determine which of the potential risk
factors identified in the descriptive
studies are actually relevant in the
spread of disease
– Cross-sectional studies
Types of Epidemiological Studies

Analytical studies are designed to
determine which of the potential risk
factors identified in the descriptive
studies are actually relevant in the
spread of disease
– Prospective studies
• Cohort groups
Types of Epidemiological Studies

Experimental studies
– To judge
• Cause and effect relationship of the risk factors
• Cause and effect relationship of preventative
factors and the development of disease
– To assess
• value of a particular intervention or treatment
– should be compared against a placebo or a
treatment of known effectiveness
– should be a double-blind study
Identifying the Source of an
Epidemic

Precise identification of an infectious
agent is important
– Different disease agents may produce the
same symptoms
– A single agent may produce a number of
manifestations
– May need identification not only of the
organism but also the precise strain of the
organism
Identifying the Source of an
Epidemic

Precise identification may involve
– Bacteriophage typing
– Antibiogram
– Surface antigen identification
– Genetic analysis
Identifying the Source of an
Epidemic

Precise identification may involve
– Bacteriophage typing
Identifying the Source of an
Epidemic

Precise identification may involve
– Antibiogram
Identifying the Source of an
Epidemic

Precise identification may involve
– Surface antigen identification
Identifying the Source of an
Epidemic

Precise identification may involve
– Genetic analysis
Infectious Disease Surveillance


National Disease Surveillance
Worldwide Disease Surveillance
– Weekly Epidemiological Record
Infectious Disease Surveillance

Worldwide Disease Surveillance
– World Health Organization
• Provide worldwide guidance in the field of
health
• To set global standards for health
• To co-operatively strengthen national public
health programs
• To develop and transfer appropriate health
technology
• Weekly Epidemiological Report
– http://www.who.int/wer/en/
Infectious Disease Surveillance

National Disease Surveillance
– National Centers for Disease Control and
Prevention (CDC)
• Morbidity and mortality weekly report
– http://www.cdc.gov/mmwr/
• 58 notifiable diseases
– State Public Health Laboratory
– Local Public Health Agencies
– Other components
• hospital labs, physicians, public schools etc.
Infectious Disease Surveillance

National Disease Surveillance
– National Centers for Disease Control and
Prevention (CDC)
• Morbidity and mortality weekly report
• 58 notifiable diseases
Infectious Disease Surveillance

National Disease Surveillance
State Public Health Laboratory
Infectious Disease Surveillance

National Disease Surveillance
– Local Public Health Agencies
Infectious Disease Surveillance

National Disease Surveillance
– Other components
• hospital labs, physicians, public schools etc.
Trends in Disease

Reduction and eradication of disease
– Small pox
– Polio

Emerging Diseases
– Newly recognized diseases
• Ebola virus
• Hantavirus
• HIV
– Familiar diseases that are re-emerging
Trends in Disease

Reduction and eradication of disease
– Small pox
– Polio
Polio

A disease that has been reduced in
incidence
Trends in Disease

Emerging Diseases
– Newly recognized diseases
• Ebola virus
• Hantavirus
• HIV
– Familiar diseases that are re-emerging
• Tuberculosis
• Syphilis
Trends in Disease

Emerging Diseases
– Microbial evolution
– Complacency and breakdown of public
health infrastructure
– Changes in human behavior
– Advances in technology
– Population expansion
– Development
Trends in Disease

Emerging diseases (continued)
– Mass distribution and importation of food
– War and civil unrest
– Climate changes
Infectious Disease Control in
Special Situations

Day-care Centers
– Infants in diapers
– Young clients are oblivious to hygiene rules
– Young children have not acquired immunity
to many common illnesses
– Day care staff need to be aware of
sanitation procedures to prevent
transmission of disease
Infectious Disease Control in
Special Situations

Hospitals and Nosocomial infections
– 2% to 10% of all hospitalized patients
acquire nosocomial infections
– At least 1/2 of all hospital infections
– Many sources of infection
•
•
•
•
Therapeutic procedures
Hospital environment
Hospital personnel
Patients own normal flora
Infectious Disease Control in
Special Situations

Hospitals and Nosocomial infections
– Types of bacteria
• Enterococcus
• E-coli
• Pseudomonas- found in humidifiers of
ventilators, sinks and toilets
• Staphylococcus aureus
• Other Staphylococci
• Streptococcus pyogenes
Infectious Disease Control in
Special Situations

Hospitals and Nosocomial infections
– Prevention of infections
• Use of Universal precautions or body
substance isolation procedures
• Compliance with these precautions
– Infection control practitioner
– Hospital epidemiologist
– Infection control committee