Epidemiology: Patterns, Transmission, and Tracking of Infectious Diseases

Epidemiology

Definition and Scope

Epidemiology is the study of when and where diseases occur, how they are transmitted, and their causes. It is a fundamental discipline in microbiology, focusing on the distribution and determinants of health-related events in populations.

• Etiology: The cause of a disease (e.g., SARS-CoV-2 is the etiology of COVID-19).

• Transmission: The mechanism by which a disease spreads (e.g., Zika virus is transmitted by mosquitos; COVID-19 is transmitted via airborne respiratory particles).

Key Epidemiological Terms

• Morbidity: Refers to illness or disease within a population.

• Incidence: The number of new cases of a disease in a population during a specific time period.

• Prevalence: The total number of cases (new and existing) in a population at a given time point.

• Mortality: The number of deaths caused by a disease in a population.

Example: Incidence measures new HIV infections per year, while prevalence measures all active HIV infections at a given time.

Patterns of Disease Incidence

Understanding how diseases appear and spread in populations is crucial for public health.

• Sporadic: Occasional occurrence of disease in a population (e.g., plague).

• Endemic: Disease constantly present in a population or region (e.g., malaria in Nigeria; syphilis in the United States).

• Epidemic: Disease affects a large part of the population in a particular region (e.g., seasonal influenza).

• Pandemic: Epidemic that spreads across multiple continents (e.g., HIV, COVID-19).

Other Patterns of Infectious Disease

• Nosocomial Infectious Disease: Acquired while staying in a hospital or healthcare facility (e.g., Clostridioides difficile).

• Emerging Infectious Disease: New to the human population or has increased in prevalence in the past 20 years (e.g., Ebola, COVID-19).

• Reemerging Infectious Disease: Disease increasing in frequency after a period of decline (e.g., drug-resistant malaria, tuberculosis, syphilis).

Example: Measles reappearing in the U.S. after elimination is a reemerging disease.

Transmission of Infectious Diseases

Reservoirs and Routes

For a disease to spread, the pathogen must have a reservoir and a transmission route.

• Reservoir: Where the pathogen survives (can be living hosts or nonliving environments such as soil, water, or objects).

• Transmission Route: How the pathogen moves to a new host.

• Host Susceptibility: Factors that make a host more likely to get sick (e.g., immunity, age).

• Portal of Entry: How the pathogen enters the new host (e.g., respiratory, digestive, reproductive systems).

• Portal of Exit: How the pathogen leaves the host (e.g., phlegm, body fluids).

Modes of Transmission

• Direct Transmission: Person-to-person contact, including vertical transmission (mother to fetus).

• Droplet Transmission: Respiratory droplets that travel less than 3 feet.

• Indirect Transmission: Via fomites (inanimate objects), vehicles (air, food, water), or vectors (insects).

• Vehicle Transmission: Pathogen carried by air (airborne particles), food (gastrointestinal pathogens), or water.

• Vector Transmission:

  • Biological Vector: Insect is infected and transmits pathogen via bite (e.g., mosquito with malaria).

  • Mechanical Vector: Pathogen carried externally (e.g., fly with pathogen on legs).

Tracking Infectious Diseases

Reporting and Surveillance

Cases of infectious diseases must be reported by healthcare workers to public health agencies for monitoring and control.

• Local Agencies: Marin County Health and Human Services

• State Agencies: California Department of Public Health

• National Agencies: Centers for Disease Control and Prevention (CDC)

• International Agencies: World Health Organization (WHO)

• Reports: Morbidity and Mortality Weekly Reports (MMWR)

Summary Table: Patterns of Disease Incidence

Pattern	Description	Example
Sporadic	Occasional occurrence	Plague
Endemic	Constantly present	Malaria in Nigeria, Syphilis in US
Epidemic	Large part of population in a region	Seasonal Influenza
Pandemic	Multiple continents	HIV, COVID-19
Emerging	New or increasing in prevalence	Ebola, COVID-19
Reemerging	Increasing after decline	Drug-resistant TB, Measles
Nosocomial	Acquired in healthcare facility	C. difficile

Formulas in Epidemiology

• Incidence Rate: $\text{Incidence Rate} = \frac{\text{Number of new cases in a time period}}{\text{Population at risk}}$

• Prevalence Rate: $\text{Prevalence Rate} = \frac{\text{Total number of cases at a given time}}{\text{Population}}$

• Mortality Rate: $\text{Mortality Rate} = \frac{\text{Number of deaths due to disease}}{\text{Total population}}$

Additional info: Academic context and definitions were expanded for clarity and completeness. Examples and formulas were added to make the notes self-contained and suitable for exam preparation.