Disease Terminology and Epidemiology

Basic Concepts in Infectious Disease

Understanding infectious disease and epidemiology is fundamental in microbiology. Infectious diseases are illnesses caused by pathogens, while epidemiology involves monitoring and controlling disease occurrence to promote public health.

• Pathogens: Include prions, viruses, bacteria, protozoans, helminths, and fungi.

• Opportunistic pathogens: Cause disease only in weakened hosts.

• True pathogens: Can cause disease in healthy hosts.

• Emerging pathogens: Newly identified or expanding in distribution (e.g., SARS-CoV-2).

• Reemerging pathogens: Previously controlled but now resurging (e.g., antibiotic-resistant bacteria).

• Zoonotic diseases: Transmitted from animals to humans.

• Communicable diseases: Transmit from human to human; contagious diseases are easily spread.

• Acute vs. Chronic diseases: Acute have rapid onset; chronic progress slowly.

Sources and Transmission of Pathogens

Pathogens originate from various sources, known as reservoirs, and are transmitted through different modes:

• Reservoirs: Natural habitats (animate or inanimate) of pathogens.

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

• Exogenous source: Pathogen from outside the host.

Stages of Infectious Disease

Infections progress through five general stages:

1. Incubation period

2. Prodromal phase

3. Acute phase

4. Period of decline

5. Convalescent phase

Infectivity: Ability to establish infection. Pathogenicity: Ability to cause disease. Virulence: Severity of disease.

The Epidemiological Triangle

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

• Host factors: Health, age, sex, lifestyle, genetics.

• Etiological agent: Type of microbe (bacteria, virus, etc.).

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

Host–Microbe Interactions and Pathogenesis

Normal Microbiota and Pathogenicity

Host–microbe interactions are dynamic. Normal microbiota colonize various body sites and usually do not cause harm. Pathogens have adaptations for host interaction and can cause disease, especially if microbiota balance is disrupted (dysbiosis).

• Opportunistic pathogens: Cause disease under certain conditions (e.g., weakened immunity).

• Tropism: Pathogen preference for specific host tissues.

Virulence and Virulence Factors

Virulence factors are mechanisms that help pathogens overcome host defenses, adhere, invade, and damage host tissues.

• Adhesion (pili, fimbriae)

• Invasion (enzymes, flagella)

• Immune evasion (capsules, antigenic variation)

• Toxin production (endotoxins, exotoxins)

• Nutrient acquisition (siderophores, iron-binding proteins)

Toxins as Virulence Factors

• Endotoxins: Lipopolysaccharide (LPS) from Gram-negative bacteria; released upon cell death; can cause septic shock.

• Exotoxins: Soluble proteins secreted by bacteria; classified by target (neurotoxins, enterotoxins, etc.).

Five Steps to Infection

To establish infection, pathogens must:

1. Enter the host

2. Adhere to host tissues

3. Invade and obtain nutrients

4. Replicate while evading immune defenses

5. Transmit to a new host

Portals of Entry and Exit

Pathogens use specific portals to enter and exit the host, often the same for both processes.

• Respiratory mucosa

• Gastrointestinal mucosa

• Skin

• Urogenital tract

• Ocular, otic, parenteral, transplacental

Immune Evasion Strategies

• Intracellular lifestyle (e.g., Mycobacterium tuberculosis)

• Latency (e.g., herpesviruses)

• Antigenic masking, mimicry, and variation

• Interference with phagocytosis

• Immune suppression (e.g., proteases, interference with cytokines)

Innate and Adaptive Immunity

Overview of Immune Responses

The immune system eliminates antigens through two main branches: innate and adaptive immunity. Both recognize pathogens, eliminate invaders, and distinguish self from non-self.

• Innate immunity: Inborn, nonspecific, rapid response.

• Adaptive immunity: Specific, develops over time, has memory.

First-Line Defenses

First-line defenses prevent pathogen entry and include mechanical, chemical, and physical barriers.

• Mechanical: Flushing (tears, urine), mucociliary escalator.

• Chemical: Lysozyme, stomach acid, antimicrobial peptides (AMPs), fatty acids.

• Physical: Skin, mucous membranes.

Leukocytes and Lymphoid Tissues

Leukocytes (white blood cells) are essential for immune responses. Primary lymphoid tissues (bone marrow, thymus) produce and mature leukocytes; secondary tissues (lymph nodes, spleen, MALT) filter lymph and sample antigens.

Types of Leukocytes

• Granulocytes: Neutrophils, eosinophils, basophils, mast cells.

• Agranulocytes: Monocytes (macrophages), dendritic cells, lymphocytes (B, T, NK cells).

Cytokines and Complement System

• Cytokines: Signaling proteins for immune cell communication; can be diagnostic markers.

• Complement system: Series of proteins that enhance phagocytosis (opsonization), promote inflammation, and cause cytolysis.

Inflammation and Its Phases

Inflammation is a key innate response to tissue injury or infection, with three main phases:

1. Vascular changes: Increased blood flow and vessel permeability.

2. Leukocyte recruitment: Neutrophils and monocytes migrate to the site.

3. Resolution: Inflammation subsides, tissue repair begins.

Fever as an Innate Response

• Enhances interferon activity

• Increases phagocyte efficiency

• Limits pathogen growth

• Promotes tissue repair

Immune System Disorders

Immunodeficiencies

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

• Secondary (acquired): Result from aging, infections, medical interventions, or systemic disorders.

Autoimmunity and Hypersensitivities

• Autoimmunity: Immune attack against self-tissues (e.g., lupus, rheumatoid arthritis).

• Hypersensitivities: Inappropriate immune responses classified as:

Transplantation and Graft Rejection

• Autografts: From self; no rejection.

• Isografts: From identical twin; minimal rejection.

• Allografts: From same species; risk of rejection.

• Xenografts: From different species; high rejection risk.

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

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