epidemiology

Thinking about Microbiology

• The first formal process for curbing the spread of infection was instituted in 1377 in Dubrovnik, Croatia. This process involved detaining travelers from affected regions until their health was confirmed.

• Initially, the detainment period was 30 days, but it was later extended to 40 days (quaranta giorni), which is the origin of the term "quarantine."

The Role of Epidemiology in Controlling Disease

• Epidemiology is defined as the study of the distribution of disease in populations and the factors related to that distribution.

• The ultimate goal of epidemiology is to reduce disease incidence in populations.

Learning Objectives

• Distinguish between:
    - Communicable diseases: Diseases that can be transmitted from one person to another.
    - Noncommunicable diseases: Diseases that cannot be directly transmitted between individuals.

• Differentiate between the types of disease occurrence:
    - Epidemic: An increase in the usual number of cases.
    - Sporadic: Occasional occurrence at low numbers, often not communicable.
    - Endemic: Constant presence of a disease within a particular region.
    - Pandemic: An epidemic that occurs over a large geographic area, affecting multiple countries or continents.

• Explain the components of the chain of infection and strategies to break this chain.

• Determine incidence and prevalence rates in population health.

• Describe the eradication of smallpox as a successful case study in infectious disease control.

Vocabulary

• Communicable disease: An infectious disease transmissible (directly or indirectly) from one person to another.

• Noncommunicable disease: Diseases that generally cannot be spread from one person to another.

• Notifiable disease: Diseases that must be reported to health authorities upon diagnosis.

• Epidemic:
    - Common source epidemic: Resulting from a single source of exposure.
    - Propagated epidemic: Spread from person to person.

• Index case: The first case of reported disease in a population.

• R0 (Basic reproduction number): A metric used to indicate the contagiousness of an infectious disease.

• Other terms include endemic, pandemic, sporadic, incidence rate, prevalence rate, virulence factor, opportunistic pathogen, reservoir, zoonosis, infectious dose, portal of entry/exit, direct/indirect contact, vertical transmission, fomite, vehicle, biological/mechanical vector, healthcare-associated infections (HAIs), and herd immunity.

How Good Are Our Statistics About Disease?

• Discusses the distinction between infectious and non-infectious diseases and communicable versus non-communicable diseases.

• Emphasizes the significance of notifiable diseases and the role of reputable organizations such as the Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO) in collecting and disseminating disease statistics.

Historical Context and Statistics of Major Pandemics

• Black Death (Bubonic Plague): 1347-1351, Death toll: approx. 200 million.

• Smallpox: 1520, Death toll: estimated 56 million.

• Spanish Flu: 1918-1919, Death toll: 40-50 million.

• Plague of Justinian: 541-542, Death toll: 30-50 million.

• HIV/AIDS: 1981-present, Death toll: 25-35 million.

• COVID-19: Ongoing since 2019, Death toll as of March 1, 2023: approx. 6.9 million.

• Provides context on the effects of the bubonic plague on Europe's population. It was estimated to have wiped out 30-50% of the population and took more than 200 years for recovery.

• Smallpox: 90% of Native Americans were killed by smallpox; led to 400,000 deaths annually in Europe in the 1800s.

• The first-ever vaccine was developed for smallpox, and the death toll remains debated concerning its historical implications.

Disease Rates and Statistics

• Reported cases and trends in various infectious diseases over specific years and populations, including case examples like Lyme disease and timelines of diseases such as measles.

• Breakdown of leading causes of death among persons aged 25-44 in the USA between 1987-2000, highlighting the mortality rates of unintentional injuries, cancer, heart disease, and more.

Nationally Notifiable Infectious Diseases (2017)

• Comprehensive list including but not limited to:
    - Anthrax, Arboviral diseases, Babesiosis, Hepatitis, HIV, Botulism, and more.

Disease Distribution

• Epidemic: An increase in usual case numbers.
    - Common source: E.g. foodborne illnesses.
    - Propagated: Spread from person-to-person, resulting in sustained infections.

• Pandemic: A worldwide epidemic.

• Endemic: Constant presence in a given area.

• Sporadic: Low, occasional occurrences that aren't continuously spread.

Basic Reproduction Number (R0)

• Defines the expected number of people that one infected person will infect.

• Listed R0 values for several infectious diseases indicate their transmissibility:
    - Hepatitis C: R0 = 2
    - Ebola: R0 = 2
    - HIV: R0 = 4
    - SARS: R0 = 4
    - Mumps: R0 = 10
    - Measles: R0 = 18

Incidence and Prevalence Rates

• Incidence Rate: Frequency of new disease cases over time in a defined population, calculated by the formula:
    $ext{Incidence} = rac{ ext{number of new cases}}{ ext{population}}$

• Prevalence Rate: Snapshot of the total number of cases (both old and new), represented as:
    $ext{Prevalence} = rac{ ext{number of cases}}{ ext{population}}$

Case Study Examples

• Example: Genital herpes diagnosed in a community college health center over one year (2025) including rates of incidence and prevalence.

• A) Calculate the incidence considering 2000 infections with 1000 recurrent cases.

• B) Determine the prevalence on Jan 1, 2026, revealed that 2500 students were infected.

The Chain of Infection

• Components include:
    - Susceptible Host: Individuals at risk of infection.
    - Pathogen: Infectious agent responsible for disease.
    - Portal of Entry: The route by which a pathogen enters a host.
    - Route of Transmission: The means by which the pathogen spreads.
    - Reservoir: Where the pathogen survives and multiplies.
    - Portal of Exit: The path by which the pathogen leaves the host.

Infectious Agents

• Pathogens possess virulence factors, which are gene products enabling them to:
    - Avoid host defenses (leading to infection).
    - Cause damage (resulting in disease beyond normal microbiota).

• Opportunistic Pathogens: Microorganisms that typically cause diseases in immunocompromised hosts.

Reservoirs of Infection

• The environments conducive to pathogen survival and spread include:
    - Humans: Infected individuals, carriers, or temporarily infected persons.
    - Animals: Accidental hosts (zoonosis) such as in cases of anthrax or rabies.
    - Nonliving: Environments like soil or water wherein bacteria like tetanus thrive.

• Note: 70% of emerging infectious diseases originate from animal contact due to human expansion into wildlife habitats.

Transmission of Infection

• Portal of Exit: The mechanism through which pathogens exit the host.

• Typical Portals of Entry: Mechanisms include:
    - Infectious dose: Minimum organisms needed to establish infection.
    - Contact transmission methods:
      - Vehicle transmission: Running through food, water, and blood.
      - Vectors: Arthropods (e.g., mosquitoes, ticks) providing transmission routes.

Contact Transmission

• Direct Contact: Physical interaction with infected fluids/lesions.

• Droplet Transmission: Spread through mucus droplets over short distances <1m.

Vertical Transmission

• Defined as transmission from mother to fetus, detailing the estimated number of AIDS diagnoses in children aged 13 and younger in the U.S. from 1992-2010.

Indirect Contact

• It involves an intermediate mechanism for pathogen spread, such as through infected aerosols or surfaces.

Indirect Contact Mechanisms

• Fomites: Inanimate agents (e.g., tissues, surfaces) through which disease spreads.

• Vehicles: Substances capable of transmitting pathogens, including contaminated food or water.

• Vectors: Living organisms (like arthropods) acting as carriers of infections; this includes biological vectors (where pathogens multiply) and mechanical vectors (where pathogens are physically transmitted).

Potential Rise in Local Transmission

• Case studies regarding malaria and the implications of climate change emphasizing risks in infection rates.

Susceptibility to Infection

• Factors affecting how susceptible a person is to infection include:
    - Previous exposures and vaccinations (influencing herd immunity).
    - Increased susceptibility due to factors such as immune suppression or burns.

Vaccination Coverage and Disease Outbreaks

• Case example illustrating the correlation between vaccination coverage rates and measles outbreaks in Los Angeles County.

Healthcare-Associated Infections (HAI)

• Defined as infections that occur in healthcare settings where patients are at increased risk.
    - Accounts for 5-15% of patients; approximately 1/10 die of these infections.

• Statistics: In American hospitals, HAIs lead to 1.7 million infections and roughly 99,000 deaths yearly.

• The breakdown of HAIs:
    - 32% urinary tract infections
    - 22% surgical site infections
    - 15% pneumonia
    - 14% bloodstream infections

Infection Control Strategies

• Questions to ascertain infection control include:
    - Identical isolates among affected patients?
    - Contact among patients?
    - Shared medical pathways?
    - If airborne, issues with ducts?
    - Testing health workers/items for pathogens?
    - Maintenance of standard infection control measures?

Breaking the Chain of Infection

• Strategies involve:
    - Elimination of pathogen reservoirs.
    - Treatment of infected individuals.
    - Preventing transmission via community awareness.
    - Reducing susceptible populations through immunizations and other methods.

Elimination and Comparison of Smallpox

• Smallpox's eradication was successful due to:
    - Humans being the only reservoir.
    - No latent infections.
    - No variation in the antigen.
    - Recovery guarantees immunity against future infections.
    - Successful vaccination strategies and surveillance methods.

• Comparison with other diseases indicates differing challenges in eradication.

The Viral 'Hit List' and Challenges in Eradication

• Comparison of measles, polio, and other infectious diseases regarding features influencing eradication complexity.
    - Smallpox:
      - Mode: Respiratory droplets
      - R0: 5-7
      - Community immunity: 80%-85%
    - Polio:
      - Mode: Fecal-oral, oral-oral
      - R0: 4-13
      - Community immunity: 75%-92%
    - Measles:
      - Mode: Respiratory droplets
      - R0: 12-18
      - Community immunity: 92%-94%