Lecture Notes: Infectious Diseases and Transmission

Key Topics Covered:

• Definitions
• Stages of Infectious Disease
• Classification of Infectious Diseases
• Reservoirs of Infection
• Pathogen Entry and Exit
• Modes of Transmission

Disease Definitions

• Infection: When a contaminating organism evades the body's defenses and multiplies.
• Disease: An infection that alters normal body functions, resulting in symptoms and signs.
• Infectious Disease: Caused by bacteria or viruses and can be transmitted to others.
• Symptoms vs. Signs:
  • Symptoms: Subjective characteristics felt by the patient (e.g., pain).
  • Signs: Objective manifestations observed or measured by others (e.g., fever).
  • Asymptomatic: Lack of symptoms but may still show signs and be contagious.

Stages of Infectious Disease

1. Incubation Period: Time between infection and first symptoms/signs.
2. Prodromal Period: Short duration with mild symptoms.
3. Illness Phase: Most severe stage with evident symptoms and signs.
4. Decline: Gradual reduction of symptoms as recovery begins.
5. Convalescence: Recovery phase with absence of symptoms.

• Infectious agents can be contagious during any of these stages.

Classification of Infectious Diseases

• Acute Disease: Rapid onset, short duration (e.g., common cold).
• Chronic Disease: Develops slowly, may be recurrent (e.g., hepatitis).
• Latent Disease: Pathogen remains inactive before becoming active again (e.g., shingles).

Reservoirs of Infection

• Human Reservoirs: Principal living reservoir; includes symptomatic and asymptomatic carriers.
• Animal Reservoirs: Zoonosis; diseases naturally spread from animals to humans.
• Non-living Reservoirs: Soil, water, and food contaminated by feces or urine. Examples include tetanus and botulism.

Pathogen Entry

• Portal of Entry: Pathogen entry sites.
  • Skin: Through cuts or abrasions.
  • Mucous Membranes: Respiratory or GI tract.
  • Placenta: Some pathogens can cross (e.g., listeria).
  • Parenteral Route: Mechanical deposition (e.g., needles).

Modes of Transmission

• Contact Transmission:

  • Direct Contact: Physical touch, sexual intercourse.
  • Indirect Contact: Through fomites (inanimate objects).
  • Droplet: Inhalation of respiratory droplets from coughing/sneezing.

• Vehicle Transmission:

  • Airborne: Pathogens travel through the air (e.g., measles).
  • Foodborne: Pathogens in contaminated food.
    • Intoxication: Ingestion of exotoxins.
    • Infection: Consumption of living organisms.
  • Waterborne: Pathogens in contaminated water (e.g., Cryptosporidium).

• Vector Transmission:

  • Biological Vectors: Organism serves as host (e.g., malaria via mosquitoes).
  • Mechanical Vectors: Transmits organisms by contact (e.g., flies).

Pathogen Exit

• Pathogens exit through similar portals as entry and are transmitted through body excretions such as blood, urine, and feces.

These notes cover part one of the chapter 13 lecture on infectious diseases and transmission.