Introduction to Epidemiology

Course Aim

• Provides a foundation of topics in epidemiology
• Examines infectious diseases, chronic diseases, and general health
• Considers ethical and professional standards
• Students learn from real-world health problems
• Demonstrates how epidemiology is used to understand, prevent, and treat "health states."

Course Assessments

• First quiz at the end of the 4th week of the course
• Mid-term assessment by the end of the 9th week of the course
• Second quiz by the end of the 13th week of the course
• Final exam after the end of the 15th week of the course

Epidemiology Definition

• The study of the distribution and determinants of health-related states or events in specified populations
• Includes the application of this study to the control of health problems

Key Aspects of Epidemiology

• Distribution: Determines who is affected by health issues, when they occur, and where
• Determinants: Investigates the causes, risk factors, and other influences on health outcomes
• Application: Aims to control and prevent health problems in communities

Purposes of Epidemiology

• Investigate the nature and extent of health-related phenomena in the community, and to identify priorities
• Study the natural history and prognosis of health-related problems
• Identify causes and risk factors
• Recommend and assist in the application or evaluate best interventions, like preventative and therapeutic measures
• Provide the basis for public policy

Epidemiologic Triad

• Agent: An organism that causes or contributes to produce infection
• Causative agents for infections include bacteria, viruses, fungi, and parasites
• Host: A susceptible human or animal that harbors and nourishes a disease-causing agent
• Physical, psychological, and lifestyle factors influence the host's susceptibility and response to an agent
• Environment: Refers to all external factors surrounding the host that influence vulnerability or resistance

Chain of Infection

• Describes the spread of an infection within a community
• The spread occurs by a series of interconnected steps that describe how a pathogen moves about

Chain of Infection: Components

• Infectious Agent: The pathogen (germ) that causes diseases
• Reservoir: Places in the environment where the pathogen lives, including people, animals, insects, medical equipment, soil, and water
• Portal of Exit: The way the infectious agent leaves the reservoir, such as through open wounds, aerosols, and body fluids
• Mode of Transmission: How the infectious agent is passed, either through direct or indirect contact, ingestion, or inhalation
• Portal of Entry: The way the infectious agent enters a new host through broken skin, the respiratory tract, mucous membranes, and catheters
• Susceptible Host: Any person, especially those receiving healthcare, immunocompromised, or with invasive medical devices

Infectious Agents: Bacteria

• Single-celled microorganisms that come in different shapes like cocci (round), bacilli (rod-shaped), and spirilla (spiral)
• Bacteria can enter the body through routes like the mouth, eyes, nose, or open wounds
• Examples of bacterial infections include strep throat, tuberculosis, and E. coli infections

Infectious Agents: Viruses

• Smaller than bacteria and require a host cell to replicate
• Viruses contain genetic material (DNA or RNA) surrounded by a protein coat
• Viruses invade host cells, insert their genetic material, and force the cell to produce more viruses
• Common viral infections: Influenza, and COVID-19

Infectious Agents: Fungi

• Can be single-celled (like yeasts) or multi-cellular organisms found in the environment
• Fungal infections often affect the skin, causing conditions like ringworm
• More serious fungal infections affect the lungs or bloodstream

Infectious Agents: Parasites

• Organisms that live off other organisms
• Parasitic infections are transmitted through contaminated water, food, or insect bites

Reservoirs of Infection

• Defined as any person, animal, arthropod, plant, soil or substance in which an infectious agent lives and multiplies
• The infectious agent depends on this reservoir primarily for survival
• Agent reproduces in such a manner that it can be transmitted to a susceptible host

Human Reservoir: Case

• Refers to a person with an obvious infection presenting clinical symptoms
• The infection may be clinical (manifested), subclinical (not clinically manifested), or latent

Human Reservoir: Carrier

• An infected person or animal that harbors a specific infectious agent without visible clinical signs
• Serves as a potential source of infection for others

Types of Carriers

• Incubatory: Individuals that are able to transmit the infectious agent during the incubation period before displaying symptoms themselves
• Convalescent: People continue to shed the disease agent during the period of recovery from the illnesss
• Healthy/Asymptomatic: Individuals that harbor and transmit a pathogen without ever developing symptoms of the disease
• Chronic: Individuals that continue to harbor and excrete the infectious agent for long periods, sometimes indefinitely
• Temporary: Those that shed the infectious agent for short periods of time

Animal Reservoirs

• Animals can be a source of infection either as carriers or cases
• Some diseases transmitted naturally between animals may accidentally infect man (zoonoses)
• Infection may occur during working in a farm environment

Examples of Zoonosis

• Dogs and cats: rabies
• Horses: tetanus

Nonliving Reservoir

• Environments or objects where pathogens can survive and potentially be transmitted to hosts
• Pathogens typically do not multiply in these environments
• Soil, water, and inanimate things may act as a reservoir infectious diseases

Portal of Exit Definition

• The route by which an agent escapes from the host

Common Portals of Exit

• Respiratory tract: Organisms find the exits with respiratory discharges, as in sneezing and coughing
• Gastrointestinal tract: Organisms find the exit in feces, the cholera organism also exists in emesis
• In blood: maternal-fetal circulation, blood transfusion, biting arthropods
• Skin and mucous membranes: as in skin lesion, infected wounds, skin eruption

Modes of Transmission

• Droplet infections (inhalation or airborne)
• Ingestion infection (food-borne infection)
• Arthropod-borne infections (vector-borne infection)
• Contact infection (percutaneous infection)
• Vertical infection

Direct Droplet Transmission

• Droplets contain infectious agents that are expelled when an infected person coughs, sneezes, talks, or during certain medical procedures
• These pathogens have a short infectious distance, typically 1 meter, and infect others through eyes, nose, or mouth
• EX: Influenza and whooping cough

Indirect Transmission: Airborne Transmission

• Some pathogens can remain suspended in the air for long periods and travel longer distances
• The size of these pathogens is less than 5 microns, allowing for an easier inhalation
• Measles and tuberculosis are airborne

Ingestion Infection

• Foodborne infections that occur when contaminated food or drinks are consumed, resulting in inflammation
• Symptoms can include diarrhea, nausea, vomiting abdominal pain, cramps
• EX: Norovirus, Salmonella

Arthropod Borne Infection

• Biological vectors: the pathogen reproduces or develops within the vector before transmission, malaria through mosquitoes
• Mechanical vectors: the vector simply carries the pathogen without it reproducing, flies carrying Shigella bacteria

Contact Infections

• Direct Transmission: Person to Person contact that occurs when an infected person directly transmits pathogens to another through physical contact, touching, and kissing
• EX: scabies
• Percutaneous Transmission: Derived from the Latin meaning "by way of the skin", infections that occur when pathogens enter through needle punctures or other sharp injuries.

Vertical Infection

• Passage of disease-causing agents (pathogens) from mother to child during pregnancy, childbirth, or shortly after birth

Portal of Entry

• Openings or routes that allow pathogens to access a susceptible host for infectious diseases

Respiratory Tract

• Common portal of entry for many pathogens including the nose, mouth, and lungs
• Airborne pathogens can be inhaled through the nose or mouth and travel into the lungs

Gastrointestinal Tract

• The digestive system provides another major entry point for pathogens: mouth, esophagus, stomach, intestines
• Infectious agents can enter through ingestion of contaminated food or water

Skin and Mucous Membranes

• Pathogens can enter through breaks in the skin (cuts, wounds, insect bites) through the eyes, nose, or mouth
• Direct contact with contaminated surfaces or bodily fluids can allow microbes to penetrate compromised skin or mucous membranes.

Urogenital Tract

• The urinary and reproductive systems can serve as portals of entry from the urethra and vagina
• Sexually transmitted infections enter through the genitourinary tract.

Other Portals

• Placenta (mother to fetus transmission)
• Injection/inoculation (e.g. needlestick injuries)
• Medical devices (e.g. catheters, tubes)

Susceptible Host

• An individual who is at risk of becoming infected by a pathogen due to a lack of immunity or resistance against that infectious agent
• It is the final link in the chain of infection and plays a crucial role in disease transmission and spread

Factors Influencing Host Susceptibility

• Immune Status
• immunocompromised conditions
• immature immunity
• immune senescence
• Lack of vaccination
• Malnutrition
• Extremes of age
• Genetic factors
• Underlying health conditions
• Pregnancy
• Environmental and behavioral factors