Chapter 14 – Infection, Infectious Diseases, and Epidemiology

Introduction to Pathogens and Microbiota

This section introduces the terminology and concepts related to disease-causing organisms and the human microbiome.

• Pathogens: Parasites that cause diseases in their hosts.

• Resident microbiota: Microorganisms that remain part of the host's microbiome for most of their life; typically have a commensal relationship (one benefits, the other is unaffected).

• Transient microbiota: Microorganisms present in the body for only short periods (hours, days, or months).

• Example: Escherichia coli is mutualistic in the colon but can cause urinary tract infections if introduced into the urinary tract (an unusual site).

Transmission and Entry of Pathogens

Pathogens can enter the body through various portals and may be transmitted from animals or the environment.

• Zoonoses: Diseases transmitted from animals to humans. Examples: Anthrax, bubonic plague, rabies, Ebola.

• Portals of entry: Common entry points include the skin, gastrointestinal tract, eyes, and placenta.

• Direct deposition: Pathogens can bypass normal barriers by being deposited directly into tissues (e.g., via punctures).

• Attachment mechanisms: Bacteria use adhesins (on fimbriae, flagella, or glycocalyces) to bind to host glycoproteins; viruses use attachment proteins.

• Avirulent: Refers to organisms that are harmless and lack the ability to cause disease.

• Example: Dental plaque is a biofilm formed by bacterial adhesion.

Definitions: Morbidity, Symptoms, and Signs

Understanding the language of disease is essential for epidemiology.

• Morbidity: Any change from a state of health (not necessarily death).

• Symptoms: Subjective effects felt by the patient (e.g., pain, headache, dizziness, fatigue).

• Signs: Objective, observable effects (e.g., swelling, rash, redness, fever).

Koch’s Postulates

Koch’s postulates are a set of criteria used to establish a causative relationship between a microbe and a disease.

1. The suspected agent must be present in all cases of the disease.

2. The agent must be isolated and grown in pure culture.

3. The cultured agent must cause the disease when inoculated into a healthy host.

4. The same agent must be found in the diseased experimental host.

Virulence and Pathogenic Mechanisms

Virulence refers to the ability of a pathogen to infect and cause disease, not the severity of the disease.

• Virulence: Relative ability of a pathogen to infect a host and cause disease.

• Extracellular enzymes: Help bacteria invade deeper tissues.

• Coagulase: Causes blood clots, allowing bacteria to hide from the immune system.

• Kinase: Breaks down clots, releasing bacteria to spread.

Types of Toxins

• Exotoxins: Secreted by living pathogens; harm neighboring cells.

• Types of exotoxins:

  • Cytotoxins: Kill or disrupt function of host cells.

  • Neurotoxins: Interfere with nerve function.

  • Enterotoxins: Affect cells lining the gastrointestinal tract.

• Endotoxins: Released from Gram-negative bacteria (specifically, lipopolysaccharide from the outer membrane).

Stages of Infectious Disease

Infectious diseases typically progress through several stages:

1. Incubation period: Time between infection and appearance of symptoms; length depends on virulence, dose, immune status, pathogen reproduction rate, and site of infection.

2. Prodromal period: Mild, nonspecific symptoms; not all diseases have this stage.

3. Illness: Most severe signs and symptoms; immune system has not yet fully responded.

4. Decline: Symptoms subside as the body returns to normal.

5. Convalescence: Recovery and tissue repair.

Transmission of Infectious Agents

Pathogens can be transmitted through various means, both direct and indirect.

• Fomites: Inanimate objects that transfer pathogens (e.g., doorknobs, utensils).

• Droplet transmission: Pathogens travel less than 1 meter in respiratory droplets.

• Aerosol: A cloud of small droplets and solid particles suspended in the air, which can carry pathogens over longer distances.

Classification of Infectious Diseases

Diseases can be classified by their duration, recurrence, and mode of transmission.

• Acute: Develops rapidly, lasts a short time.

• Chronic: Develops slowly, persists or recurs over long periods.

• Latent: Pathogen remains dormant for extended periods before causing symptoms.

• Communicable: Disease can be transmitted from one host to another.

Epidemiological Terms and Patterns

Epidemiology uses specific terms to describe the occurrence and spread of diseases in populations.

Example: The 2009 H1N1 influenza outbreak was a pandemic, as it spread across multiple continents.

Additional info: Understanding these terms is crucial for tracking and controlling infectious diseases in public health.