Epidemiology of Disease Transmission

Disease Transmission Dynamics

• Epidemiology focuses on disease transmission, pathogen virulence, clinical infection courses, protective immunity duration, and human contact patterns.

Learning Objectives

• Understand disease transmission models.
• Define terms related to disease occurrence.
• Calculate and interpret attack rates to assess disease transmission.
• Describe outbreak investigation steps and identify sources using cross-tabulation.

The Epidemiologic Triad

• Infectious disease: Caused by infectious agents or their toxins, transmitted by infected persons, animals, or reservoirs, often via vectors.
• Triad relationship: Links infectious agents, hosts, transmission modes, and environments where exposure occurs.
• Human susceptibility factors: Include genetic, behavioral, nutritional, and immune factors.

Malaria Example

• Agent: Plasmodium parasite
• Host: Humans
• Environment: Mosquito-prone areas

Sources and Reservoirs of Infection

• Human reservoirs: Infected individuals, carriers
• Animal reservoirs: Zoonotic diseases (e.g., rabies)
• Environmental reservoirs: Cholera transmission (contaminated water)

Rabies

• Rabies virus is a zoonotic causative agent transmitted through animal reservoirs, like dogs.

Cholera

• Vibrio cholerae is an environmental reservoir often found in contaminated water.

Host Factors and Risk

• Table 2.1 correlates host characteristics—age, sex, race, religion, occupation, genetic profile, family background, marital status, and immunes status—with increased infectious disease risk

Modes of Transmission

• Direct: Person-to-person contact
• Indirect: Common vehicle (contaminated food), vectors (e.g., mosquitoes)

Respiratory Viruses (e.g., COVID-19)

• Spread by droplets and aerosols (exhaled during breathing, speaking, coughing, and sneezing).
• Droplets: Larger particles (>5-10 µm)
• Aerosols: Smaller particles (<5 µm)
• COVID-19 transmission: Airborne and surfaceborne.

Viral Survival Times

• Viral survival times on different surfaces (wood, glass, plastic, steel, copper, paper, latex gloves, cloth) vary, with some lasting days.

Clinical and Subclinical Disease

• Clinical: Visible disease symptoms
• Subclinical: No or minimal visible symptom; individual is still infectious
• Example: Polio virus (visible and invisible cases)

Spectrum of Disease Severity

• Classifies diseases based on their clinical severity: Inapparent, Mild, Moderate, Severe, and Fatal
• Factors influencing spread: Severity, organism's efficiency at producing disease, organism multiplication site, and host immunity/response.

Endemic, Epidemic, Pandemic

• Endemic: Habitual presence of a disease in a geographic area.
• Example: Crimean-Congo hemorrhagic fever in Pakistan.
• Epidemic: Significant increase (often sudden) above expected levels, often linked to a common source.
• Pandemic: Worldwide epidemic.

Disease Outbreaks

• An outbreak: Occurrence of a disease above expected levels in a defined area or population.
• Common vehicle outbreak: Single exposure (eating contaminated food)
• Factors in outbreak investigation: Exposure time, disease onset, incubation period

Exploring Disease Occurrence

• Investigating disease patterns: Who was affected, when did it occur, where did it occur

Immunity and Susceptibility

• Immunity: Resistance to disease
• Susceptibility: Vulnerable to disease
• Herd immunity: Large proportion of a population is immune, reduces disease spread.
• Vaccines: Promote herd immunity.

Incubation Period

• The period from infection to visible symptoms.
• Factors influencing length: Replicating time, replication site, virus dose

Middle East Respiratory Syndrome Coronavirus (MERS-CoV)

• First reported in Saudi Arabia (2012).
• Animal reservoir: Camels
• Mode of transmission: Person-to-person contact or animal contact

Outbreak Investigation (Variables)

• Time of exposure
• Time of disease onset
• Incubation period