Principles of Disease & Epidemiology

Overview

This section covers the mechanisms by which infectious diseases spread, the role of reservoirs and transmission modes, healthcare-associated infections (HAIs), emerging infectious diseases (EIDs), and the science of epidemiology. Understanding these principles is essential for controlling disease and protecting public health.

Spread of Infection

Reservoirs of Infection

A reservoir of infection is any source where disease-causing microbes persist and from which infection can be transmitted.

• Human reservoirs: Individuals may show symptoms or be asymptomatic carriers. Examples: HIV, Hepatitis, STDs, meningitis. Carriers may have inapparent infections or latent diseases.

• Animal reservoirs: Diseases transmitted from animals to humans are called zoonoses. Examples: Rabies, Lyme disease, Ebola.

• Non-living reservoirs: Soil and water can harbor pathogens. Examples: Tetanus (soil), Cryptosporidiosis, cholera, Legionella (water).

Infection Cycle

The infection cycle describes the route an organism takes from one individual to another, facilitating the spread of disease.

Modes of Transmission

• Contact transmission:

  • Direct contact: Physical contact between source and host.

  • Indirect contact: Transmission via inanimate objects (fomites).

  • Droplet transmission: Short distance (<1 meter) via sneezing or coughing.

• Vehicle transmission: Disease transmitted via water, food, air, blood, or drugs.

  • Waterborne: Contaminated (fecal) water, compromised treatment systems.

  • Airborne: Dust particles, pet dander.

  • Foodborne: Contaminated food, improper storage, undercooked food, unsanitary handling.

• Fecal-oral transmission: Pathogens from feces are ingested orally.

• Congenital transmission: Pathogens transmitted from mother to child.

• Vector transmission: Insects transmit disease.

  • Mechanical transmission: Passive transfer via body parts.

  • Biological transmission: Involves parasite life cycle within the vector.

Healthcare-Associated Infections (HAIs)

Definition and Importance

Healthcare-associated infections (HAIs) are acquired during treatment in a healthcare facility. They affect 4-5% of all hospital patients and arise from interactions among microorganisms, compromised hosts, and transmission chains.

Key Factors in HAIs

• Opportunistic pathogens: Microbes present in the hospital environment pose a risk, especially drug-resistant types.

• Compromised host: Patients with broken skin, mucous membrane damage, or suppressed immune systems are more susceptible.

• Resistance: Infection resistance may be impaired by disease, therapy, or burns.

• Routes of transmission: Direct contact from staff or other patients; indirect via fomites or airborne routes.

Microorganisms Involved in HAIs

Principal Sites of HAIs

• Urinary tract infections: 22%

• Surgical site infections: 22%

• Lower respiratory infections: 17% (high mortality rates, often related to respiratory devices)

• Gastrointestinal infections: 13% (Clostridium difficile causes 12%)

• Bloodstream infections: 10%

• Other: 16%

Control of HAIs

• Universal precautions: Reduce transmission of microbes in healthcare and residential settings.

• Standard precautions: Basic, minimum measures applied to all levels. Includes:

  • Aseptic techniques

  • Handling of contaminated materials

  • Frequent handwashing

  • Use of isolation rooms and wards

  • Use of personal protective equipment (PPE)

  • Staff education

• Transmission-based precautions: Supplement standard precautions for highly transmissible infections. Includes:

  • Contact precautions

  • Droplet precautions (short distance)

  • Airborne precautions (long distance)

Emerging Infectious Diseases (EIDs)

Definition and Identification

Emerging infectious diseases (EIDs) are new or changing diseases that show an increase in incidence or potential to increase in the near future. EIDs can be caused by viruses, bacteria, fungi, protozoa, or helminths.

• 75% of EIDs are zoonotic, mainly of viral origin, and are likely to be vector-borne.

Criteria to Identify an EID

• Distinctive disease symptoms

• Improved diagnostic techniques allow identification of new pathogens

• Local disease becomes widespread

• Rare disease becomes common

• Mild disease becomes more severe

Factors Contributing to EIDs

• Genetic recombination/evolution

• Misuse of antibiotics/pesticides

• Climate change

• Increased human transportation

• Ecological changes

• Public health failures

Epidemiology

Definition and Importance

Epidemiology is the study of where and when diseases occur and how they are transmitted in populations. It is crucial for disease control and public health planning.

• Determining etiology: Identifying the cause of a disease.

• Identifying factors in disease spread: Demographic data, socioeconomic status, common histories.

• Developing control methods: Control of reservoirs, food inspection, sewage disposal.

• Data analysis: Assembling data and graphs to outline disease incidence and test effectiveness of control measures (e.g., vaccination).

Case Reporting and Notifiable Diseases

• Case reporting: Health care workers report specified diseases to local, state, and national offices (e.g., AIDS, meningitis, STDs, tetanus). This establishes the chain of transmission.

• Nationally notifiable infectious diseases: Physicians are required to report the occurrence of certain diseases.

Centers for Disease Control and Prevention (CDC)

• Collects and analyzes epidemiological information in the U.S.

• Publishes Morbidity and Mortality Weekly Report (MMWR)

Key Terms

• Morbidity: Incidence of a specific notifiable disease.

• Mortality: Deaths from notifiable diseases.

• Morbidity rate:

• Mortality rate:

Nationally Notifiable Infectious Diseases (2017)

Summary

• Spread of Infection: Reservoirs (animal, human, non-living) and transmission modes.

• Healthcare-Associated Infections (HAIs): Microbes involved, chain of transmission, compromised host, resistance, and control measures.

• Emerging Infectious Diseases: Identification and contributing factors.

• Epidemiology: Types, case reporting, morbidity, and mortality.