10.01 Infectious Diseases

1. Definition & General Overview

Infectious Diseases

• Definition:
  Diseases caused by pathogens (organisms such as bacteria, viruses, protoctists) that can be transmitted from an infected individual to an uninfected one.
• Alternative Term:
  Communicable Diseases – Emphasizing their ability to be transmitted between hosts.

Disease Transmission

• Definition:
  The process by which pathogens spread from one host to another.
• Modes of Transmission:
  • Direct Transmission:
    • Person-to-Person Contact: Physical interactions such as touching, kissing, sexual contact.
    • Droplet Spread: Respiratory droplets from coughing or sneezing.
  • Indirect Transmission:
    • Vehicle-Borne: Through contaminated water, food, surfaces, or air.
    • Vector-Borne: Via organisms like insects (e.g., mosquitoes, ticks) that carry pathogens from one host to another.

2. Characteristics of Infectious Diseases

Host Range

• Human-Specific Diseases:
  Only affect humans (e.g., smallpox).
• Zoonotic Diseases:
  Affect both animals and humans (e.g., rabies, malaria).

Impact Severity

• Mild Diseases:
  Cause minor symptoms (e.g., common cold).
• Serious Diseases:
  Can be life-threatening or cause significant morbidity (e.g., tuberculosis, HIV/AIDS).

Duration of Infection

• Acute Diseases:
  Short-term, rapid onset and resolution (e.g., influenza, measles).
• Chronic Diseases:
  Long-lasting, often persistent conditions (e.g., tuberculosis, HIV/AIDS).

3. Pathogens & Transmission

Types of Pathogens

• Bacteria:
  Single-celled prokaryotes (e.g., Vibrio cholerae, Mycobacterium tuberculosis).
• Viruses:
  Non-cellular entities that require host cells to replicate (e.g., HIV, influenza virus).
• Protoctists (Protists):
  Eukaryotic microorganisms (e.g., Plasmodium species causing malaria).

Types of Pathogen Transmission

• Direct Contact Transmission:
  • Examples:
    • Skin contact (e.g., herpes simplex virus).
    • Exchange of bodily fluids (e.g., HIV/AIDS).
• Indirect Contact Transmission:
  • Examples:
    • Contaminated water or food (e.g., cholera).
    • Fomites (inanimate objects like doorknobs, utensils).
    • Airborne particles (e.g., tuberculosis).

Carriers

• Definition:
  Individuals who harbor pathogens without exhibiting symptoms and can unknowingly spread the disease.
• Challenges:
  • Detection: Carriers do not show symptoms, making it difficult to identify and isolate them.
  • Examples:
    • Asymptomatic carriers of Salmonella typhi (typhoid fever).
    • Individuals carrying the poliovirus without symptoms.

4. Transmission Cycle

Definition

• Transmission Cycle:
  The continuous process of pathogen spread from one host to another, ensuring the persistence of the disease within a population.

Control Methods

• Goal:
  To break the transmission cycle, preventing the spread of pathogens.
• Strategies:
  • Vaccination:
    • Function:
      • Induces immunity by exposing the immune system to antigens without causing disease.
      • Prevents pathogens from reproducing in the body, thereby halting transmission.
    • Examples:
      • Smallpox vaccine leading to eradication.
      • Polio vaccine reducing incidence globally.
  • Sanitation and Hygiene:
    • Ensures clean water supply and proper waste disposal.
  • Vector Control:
    • Reducing populations of disease-carrying insects (e.g., mosquito nets, insecticides).
  • Quarantine and Isolation:
    • Separating infected individuals to prevent disease spread.

5. Global Disease Control Efforts

Role of the World Health Organization (WHO)

• Coordination:
  • Leads international efforts to control and eradicate infectious diseases.
• Campaigns:
  • Disease Eradication:
    • Goal: Completely eliminate transmission of specific diseases globally.
    • Successful Examples:
      • Smallpox: Eradicated in 1980.
      • Rinderpest: Livestock disease eradicated in 2011.
• Ongoing Efforts:
  • Polio: Near-eradication with ongoing vaccination campaigns.
  • Malaria and HIV/AIDS: Continued research and intervention strategies.

6. Examples of Pathogens and Associated Diseases

7. Key Terminology

• Infectious Disease:
  Disease caused by a pathogen.
• Pathogen:
  An organism that causes disease.
• Disease Transmission:
  Transfer of pathogens between infected and uninfected individuals.
• Disease Carrier:
  An individual who is infected but asymptomatic and can transmit the disease.
• Transmission Cycle:
  The repeated passage of pathogens from one host to another.
• Disease Eradication:
  Complete cessation of a pathogen’s transmission, leading to zero cases globally.
• Endemic Disease:
  Disease consistently present in a population or region (e.g., TB globally; malaria in tropical areas).

8. Epidemiological Terms

• Incidence:
  The number of new cases diagnosed over a specific time period (e.g., weekly, monthly, annually).
• Prevalence:
  The total number of cases of a disease at a particular time.
• Epidemic:
  A sudden increase in cases within a specific region.
• Pandemic:
  A widespread increase in cases across multiple continents or globally.
• Mortality Rate:
  The number of deaths within a population over a given time period, typically expressed per 100,000 people to allow for comparison.

9. Additional Concepts

Reservoirs of Infection

• Definition:
  Natural habitats where pathogens live, grow, and multiply.
• Types:
  • Humans: Most common reservoirs for human-specific diseases.
  • Animals: Reservoirs for zoonotic diseases.
  • Environment: Water, soil, fomites.

Incubation Period

• Definition:
  The time between exposure to a pathogen and the appearance of symptoms.
• Variability:
  Different for each disease (e.g., influenza has a short incubation period, while HIV has a long incubation period).

Latency Period

• Definition:
  The period during which the pathogen is present in the body but not causing symptoms.
• Relevance:
  Important for diseases that can remain dormant and reactivate later (e.g., herpes simplex virus).

Reproductive Number (R₀)

• Definition:
  The average number of secondary infections produced by one infected individual in a fully susceptible population.
• Implications:
  • R₀ > 1: Disease can spread in the population.
  • R₀ < 1: Disease may eventually die out.

Herd Immunity

• Definition:
  When a significant portion of a population becomes immune to a disease, thereby providing indirect protection to those who are not immune.
• Threshold:
  Varies depending on the disease’s R₀ (higher R₀ requires a higher proportion of immune individuals).