Epidemiology of Infectious Diseases

Mr. Parag Chauhan

Introduction

  • Epidemiology is the scientific discipline that studies the distribution and determinants of health-related events in populations and applies this knowledge to control health problems.
  • When applied specifically to infectious diseases, it focuses on understanding how microorganisms spread within communities, why some populations are more vulnerable than others, and how outbreaks can be prevented or controlled.
  • Infectious diseases have shaped human history through epidemics and pandemics, influencing demographics, economies, and even geopolitics.
  • Despite major advances in medicine, they remain a significant cause of global morbidity and mortality.

Understanding the epidemiology of infectious diseases is essential for:

  • Designing preventive strategies, such as vaccination.

  • Developing effective treatment and containment measures.

  • Conducting outbreak investigations.

  • Formulating health policies and resource allocation.

 

Historical Background

  • Hippocrates (400 BC): First suggested that environmental factors influence disease.

  • Girolamo Fracastoro (1546): Proposed that diseases were spread by invisible “contagion particles.”

  • John Snow (1854): Considered the “father of modern epidemiology,” traced a cholera outbreak in London to a contaminated water pump, showing the importance of epidemiological methods.

  • Louis Pasteur and Robert Koch (late 19th century): Established the germ theory of disease.

  • 20th century onward: Development of vaccines, antibiotics, and surveillance systems led to the control of many infections (e.g., eradication of smallpox in 1980).

  • 21st century: Globalization, climate change, antimicrobial resistance, and novel pathogens like SARS, Ebola, and COVID-19 have re-emphasized the importance of infectious disease epidemiology.

 

Fundamental Concepts

The Epidemiological Triad

The occurrence of infectious disease is explained by the interaction of Agent, Host, and Environment.

  1. Agent (Pathogen):

    • Microorganisms capable of causing disease (bacteria, viruses, fungi, protozoa, helminths).

    • Properties: infectivity, pathogenicity, virulence, immunogenicity, antigenic stability.

  2. Host Factors:

    • Age, sex, genetic susceptibility, nutritional status, immunity (natural or acquired), behaviour, and co-existing conditions (e.g., HIV increasing TB risk).

  3. Environment:

    • External conditions affecting transmission (climate, sanitation, socioeconomic status, population density, migration, war, disasters).

 

Natural History of Infectious Diseases

  1. Exposure: Entry of pathogen into the host.

  2. Incubation period: Time between exposure and onset of symptoms. Varies widely (e.g., measles: ~10 days; HIV: years).

  3. Prodromal period: Early nonspecific symptoms.

  4. Clinical disease: Manifestation of typical symptoms.

  5. Outcome: Recovery, chronic carrier state, disability, or death.

 

Modes of Transmission

  1. Direct Transmission

    • Person-to-person contact (touching, kissing, sexual contact).

    • Example: Syphilis, HIV.

  2. Indirect Transmission

    • Through contaminated objects (fomites), water, food.

    • Example: Hepatitis A, cholera.

  3. Airborne Transmission

    • Via droplets (>5 µm, close range) or droplet nuclei (<5 µm, long distance).

    • Example: Tuberculosis, measles.

  4. Vector-borne Transmission

    • Biological vectors: Pathogen multiplies in vector (e.g., Plasmodium in Anopheles mosquito).

    • Mechanical vectors: Passive carriage (flies carrying Shigella).

  5. Zoonotic Transmission

    • From animals to humans (e.g., rabies, avian influenza, Nipah virus).

 

Dynamics of Infectious Disease Transmission

  1. Basic Reproduction Number (R₀):

    • The average number of secondary cases generated by one infected case in a fully susceptible population.

    • Example: Measles (R₀ ≈ 12–18), COVID-19 (R₀ ≈ 2–6 for early variants).

  2. Effective Reproduction Number (Rₑ):

    • Actual transmission rate after considering immunity and interventions.

  3. Herd Immunity:

    • When a critical proportion of the population is immune, disease spread is interrupted.

    • Threshold varies by pathogen (e.g., >90% for measles, ~70% for polio).

 

Epidemiological Measures

  • Incidence rate: New cases per population per unit time.

  • Prevalence: Proportion of the population affected at a specific time.

  • Attack rate: Proportion of exposed individuals who develop the disease.

  • Secondary attack rate: Spread among close contacts of a case.

  • Case fatality rate (CFR): Proportion of deaths among diagnosed cases.

 

Patterns of Occurrence

  1. Sporadic: Irregular cases (e.g., tetanus).

  2. Endemic: Constant presence (e.g., malaria in Africa).

  3. Epidemic (Outbreak): Sudden increase above expected levels.

  4. Pandemic: Global spread (e.g., COVID-19, influenza 1918).

 

Determinants of Infectious Diseases

  • Biological factors: Virulence, genetic mutations, drug resistance.

  • Environmental factors: Sanitation, climate, vector habitats.

  • Demographic factors: Population density, migration, urbanization.

  • Socioeconomic factors: Poverty, access to healthcare, education.

  • Behavioural factors: Travel, hygiene practices, vaccine hesitancy.

 

Epidemiological Methods

  1. Descriptive Epidemiology: Distribution of disease by person, place, time.

  2. Analytical Epidemiology: Identifies risk factors via cohort and case-control studies.

  3. Experimental Epidemiology: Clinical and field trials (vaccine efficacy).

  4. Molecular Epidemiology: Genomic sequencing for tracking pathogen evolution.

  5. Surveillance:

    • Passive: Routine reporting from healthcare providers.

    • Active: Field visits, surveys.

    • Sentinel: Selected sites for high-quality data.

    • Syndromic: Based on symptom clusters before lab confirmation.

 

Control and Prevention

Primary Prevention

  • Vaccination (measles, polio, COVID-19).

  • Vector control (mosquito nets, insecticides).

  • Safe water, sanitation, and hygiene (WASH).

  • Health education and awareness.

Secondary Prevention

  • Screening for early detection (HIV, TB).

  • Prophylactic treatment in high-risk groups.

Tertiary Prevention

  • Rehabilitation (HIV care, TB treatment adherence).

  • Preventing complications and improving quality of life.

 

Emerging and Re-emerging Infectious Diseases

  1. Emerging: New infections not previously recognised (COVID-19, Nipah virus).

  2. Re-emerging: Controlled diseases resurging due to resistance or gaps in control (TB, dengue, malaria).

  3. Drivers:

    • Global travel and trade.

    • Antimicrobial resistance (AMR).

    • Climate change is altering vector distribution.

    • Urbanisation and deforestation.

 

Modern Challenges

  • Antimicrobial resistance: Multidrug-resistant TB, MRSA, drug-resistant malaria.

  • Globalisation: Rapid spread across borders.

  • Vaccine hesitancy: Declining coverage leading to outbreaks.

  • Zoonoses: Increasing due to human-animal interactions.

  • Climate change: Expanding vector habitats (mosquito-borne diseases).