🦠Epidemiology Unit 7 – Infectious Disease Epidemiology

Key Concepts and Definitions

• Infectious diseases caused by pathogenic microorganisms (bacteria, viruses, parasites, or fungi) can spread from person to person or from animals to people
• Epidemiology the study of the distribution and determinants of health-related states or events in specified populations and the application of this study to the control of health problems
  • Descriptive epidemiology describes the distribution of diseases in terms of person, place, and time
  • Analytical epidemiology investigates the determinants and risk factors associated with diseases
• Pathogenicity the ability of an infectious agent to cause disease in a susceptible host
• Virulence the degree of pathogenicity of an infectious agent, often measured by the severity of the disease it causes
• Communicable diseases infectious diseases that can be transmitted from one person to another or from animals to humans (influenza, measles, HIV/AIDS)
• Zoonotic diseases infectious diseases that can be transmitted from animals to humans (rabies, Lyme disease, West Nile virus)
• Endemic the constant presence and/or usual prevalence of a disease or infectious agent within a geographic area or population group
• Epidemic the occurrence of disease cases in excess of normal expectancy within a specific geographic area or population during a given period

Historical Context of Infectious Diseases

• Infectious diseases have shaped human history, causing significant morbidity and mortality (Black Death, Spanish Flu, HIV/AIDS)
• Advancements in sanitation, hygiene, and public health measures (clean water, sewage systems, handwashing) have greatly reduced the burden of infectious diseases
• Vaccination one of the most successful public health interventions, has led to the eradication of smallpox and the control of many other diseases (polio, measles, rubella)
• Antibiotic discovery (penicillin) revolutionized the treatment of bacterial infections, but the emergence of antibiotic resistance poses new challenges
• Globalization and increased international travel have facilitated the rapid spread of infectious diseases across borders (SARS, COVID-19)
• Climate change and environmental factors (deforestation, urbanization) have contributed to the emergence and re-emergence of infectious diseases (Zika virus, Ebola)
• Social determinants of health (poverty, education, access to healthcare) play a significant role in the distribution and impact of infectious diseases within populations

Transmission Dynamics and Pathogen Characteristics

• Transmission the process by which an infectious agent is spread from a source to a susceptible host
• Modes of transmission include direct contact (person-to-person), indirect contact (fomites), airborne (droplets or aerosols), vector-borne (mosquitoes, ticks), and foodborne or waterborne
• Incubation period the time between exposure to an infectious agent and the onset of symptoms
  • Varies depending on the pathogen and can range from a few hours (influenza) to several years (HIV/AIDS)
• Infectious period the time during which an infected person can transmit the pathogen to others
• Basic reproduction number (R0) the average number of secondary cases generated by one primary case in a completely susceptible population
  • R0 > 1 indicates the potential for an epidemic, while R0 < 1 suggests the infection will die out
• Herd immunity the indirect protection from an infectious disease that occurs when a sufficient proportion of a population becomes immune (through vaccination or prior infection)
  • Herd immunity threshold depends on the R0 of the pathogen and the effectiveness of the vaccine
• Pathogen evolution and adaptation (antigenic drift and shift in influenza viruses) can lead to the emergence of new strains and increased virulence

Epidemiological Measures and Indicators

• Incidence the number of new cases of a disease in a population over a specified period
  • Incidence rate the number of new cases per unit of population at risk per unit of time (per 1,000 person-years)
• Prevalence the proportion of a population that has a disease at a specific point in time
  • Point prevalence the proportion of a population that has the disease at a specific point in time
  • Period prevalence the proportion of a population that has the disease during a specified period
• Mortality the number of deaths in a population over a specified period
  • Case fatality rate (CFR) the proportion of individuals diagnosed with a disease who die from that disease
• Years of potential life lost (YPLL) a measure of premature mortality, calculated by summing the years of life lost due to deaths occurring before a specified age (75 years)
• Disability-adjusted life years (DALYs) a measure of the burden of disease, expressed as the number of years lost due to ill-health, disability, or early death
• Attack rate the proportion of an at-risk population that develops the disease during a specified period (often used in outbreak investigations)
• Secondary attack rate the proportion of cases among contacts of primary cases

Surveillance Systems and Data Collection

• Surveillance the ongoing, systematic collection, analysis, and interpretation of health-related data for the planning, implementation, and evaluation of public health practice
  • Active surveillance public health officials proactively seek out cases through regular contact with healthcare providers and laboratories (resource-intensive but more complete)
  • Passive surveillance relies on healthcare providers and laboratories to report cases to public health authorities (less resource-intensive but may lead to underreporting)
• Notifiable diseases infectious diseases that healthcare providers and laboratories are legally required to report to public health authorities (varies by country and jurisdiction)
• Sentinel surveillance a system in which a selected group of healthcare providers or facilities agree to report all cases of a specific disease (useful for monitoring trends and detecting outbreaks)
• Syndromic surveillance the collection and analysis of data on clinical signs and symptoms (rather than confirmed diagnoses) to detect outbreaks early
• Electronic health records (EHRs) and health information systems can facilitate the collection, sharing, and analysis of surveillance data
• Participatory surveillance engages the public in reporting symptoms or cases through mobile apps or online platforms (Flu Near You, COVID Symptom Study)
• Wastewater surveillance the monitoring of sewage systems for the presence of pathogens (SARS-CoV-2, poliovirus) to detect outbreaks and monitor community transmission

Outbreak Investigation and Management

• Outbreak the occurrence of disease cases in excess of normal expectancy within a specific geographic area or population during a given period
• Outbreak investigation a systematic approach to identifying the source, mode of transmission, and extent of an outbreak to implement control measures
  1. Establish the existence of an outbreak by comparing current data with historical data
  2. Confirm the diagnosis and determine the case definition
  3. Identify and count cases (line listing)
  4. Orient the data in terms of person, place, and time (descriptive epidemiology)
  5. Develop hypotheses about the source and mode of transmission
  6. Evaluate hypotheses through analytical studies (cohort or case-control studies) and environmental investigations
  7. Implement control and prevention measures
  8. Communicate findings and recommendations to stakeholders and the public
• Epidemic curve a graphical display of the number of cases over time, used to understand the timing and progression of an outbreak
• Contact tracing the identification, assessment, and management of individuals who have been exposed to an infectious disease to prevent further transmission
• Quarantine the separation and restriction of movement of individuals who have been exposed to an infectious disease but are not yet ill
• Isolation the separation of individuals who are ill with an infectious disease to prevent further transmission

Prevention and Control Strategies

• Primary prevention aims to prevent the occurrence of disease through measures such as vaccination, health education, and risk reduction
• Secondary prevention focuses on early detection and prompt treatment to reduce the impact of disease (screening, early diagnosis, and treatment)
• Tertiary prevention seeks to minimize the consequences of established disease through rehabilitation and support services
• Vaccination the administration of a vaccine to stimulate the immune system to protect against a specific infectious disease
  • Herd immunity the indirect protection from an infectious disease that occurs when a sufficient proportion of a population becomes immune through vaccination or prior infection
• Infection control practices (hand hygiene, personal protective equipment, environmental cleaning) reduce the transmission of infectious agents in healthcare settings and the community
• Antimicrobial stewardship the judicious use of antimicrobials to preserve their effectiveness and prevent the development of resistance
• Risk communication the exchange of information, advice, and opinions between experts and the public to promote understanding and informed decision-making during outbreaks or public health emergencies
• One Health approach recognizes the interconnectedness of human, animal, and environmental health and promotes collaboration across sectors to address infectious disease challenges

Emerging Infectious Diseases and Global Challenges

• Emerging infectious diseases newly recognized or previously rare infectious diseases that have increased in incidence or geographic range (HIV/AIDS, SARS, COVID-19)
• Re-emerging infectious diseases previously controlled or endemic diseases that have experienced a resurgence due to factors such as antimicrobial resistance or breakdowns in public health measures (tuberculosis, measles)
• Zoonotic spillover the transmission of pathogens from animal reservoirs to humans, often facilitated by ecological, behavioral, or socioeconomic changes (Ebola, Zika virus)
• Antimicrobial resistance the ability of microorganisms to withstand the effects of antimicrobial drugs, leading to treatment failures and increased morbidity and mortality
  • Driven by factors such as overuse and misuse of antimicrobials, lack of new drug development, and the spread of resistant strains
• Climate change and environmental factors (temperature, humidity, rainfall) can influence the distribution and transmission of vector-borne and zoonotic diseases
• Health inequities and social determinants of health (poverty, education, access to healthcare) contribute to the disproportionate burden of infectious diseases among vulnerable populations
• Global health security the collective efforts to prevent, detect, and respond to infectious disease threats through international collaboration, surveillance, and capacity building
  • International Health Regulations (IHR) a legally binding agreement among WHO member states to report and respond to public health emergencies of international concern
• Pandemic preparedness and response the development and implementation of plans and strategies to mitigate the impact of widespread infectious disease outbreaks (stockpiling of medical supplies, surge capacity planning, risk communication)