Principles of Infectious Disease and Epidemiology

Basic Disease Terminology

Understanding infectious diseases and their spread is fundamental in microbiology. Infectious disease refers to illness caused by a pathogen, while epidemiology is the study and control of disease occurrence in populations to promote public health.

Koch’s Postulates

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

• The same organism must be present in every case of the disease.

• The organism must be isolated and grown in pure culture.

• The cultured organism should cause disease in a healthy host.

• The organism must be re-isolated from the experimentally infected host.

Limitations: Not all pathogens can be cultured, some only infect humans, and latent diseases complicate application.

Sources and Transmission of Pathogens

Pathogens originate from various sources:

• Reservoirs: Natural habitats (animate or inanimate) where pathogens are found.

• Endogenous: Pathogen comes from the host’s own body.

• Exogenous: Pathogen is external to the host.

Transmission modes include direct contact, airborne, vehicle (fomites, food, water), vector (biological/mechanical), and vertical (mother to child).

Stages of Infectious Disease

Infections progress through five general stages:

• Incubation period: Time between infection and earliest symptoms.

• Prodromal phase: Early, mild symptoms appear.

• Acute phase: Peak of disease; most severe symptoms.

• Period of decline: Symptoms resolve as pathogen is controlled.

• Convalescent phase: Recovery; pathogen may remain latent.

The Epidemiological Triangle

The epidemiological triangle links three factors: host, etiological agent, and environment. Disease results from the interaction of these components.

• Host factors: General health, age, sex, lifestyle, genetics.

• Etiological agent: Type of microbe (bacteria, virus, fungus, parasite, prion).

• Environmental factors: Climate, geography, vectors, water/food sources.

Host–Microbe Interactions and Pathogenesis

Five Steps to Infection

To establish infection, a pathogen must:

1. Enter the host

2. Adhere to host tissues

3. Invade tissues and obtain nutrients

4. Replicate while evading immune defenses

5. Transmit to a new host

Exotoxins are further classified by their mechanism of action (Type I, II, III).

Immune Evasion Strategies

Pathogens evade the immune system by:

• Hiding inside host cells (intracellular lifestyle, latency)

• Antigenic masking, mimicry, and variation

• Interfering with phagocytosis

• Suppressing immune responses

Innate and Adaptive Immunity

Fever

Fever is a systemic innate response that enhances immune function, inhibits pathogen growth, and promotes tissue repair.

Immune System Disorders

Immunodeficiencies

• Primary immunodeficiencies: Genetic defects affecting immune function (e.g., SCID, DiGeorge syndrome).

• Secondary immunodeficiencies: Acquired due to aging, infections, medical interventions, or systemic disorders.

Autoimmunity and Hypersensitivities

Autoimmunity is an immune attack against self-tissues, leading to chronic disorders (e.g., lupus, rheumatoid arthritis, type I diabetes).

Hypersensitivities are inappropriate immune responses classified as:

• Type I: Allergy (IgE-mediated)

• Type II: Cytotoxic (IgG/IgM-mediated)

• Type III: Immune complex (IgG/IgM complexes)

• Type IV: Delayed (T cell-mediated)

Transplantation and Immune Reactions

• Transplant rejection: Mediated by T cells recognizing foreign MHC molecules.

• Graft-versus-host disease: Donor immune cells attack recipient tissues.

• Types of grafts: Autografts, isografts, allografts, xenografts.

Summary Table: Key Concepts