Principles of Infectious Disease, Epidemiology, and Immunity

Principles of Infectious Disease and Epidemiology

Basic Disease Terminology

Understanding infectious diseases and their spread is foundational in microbiology. Epidemiology is the study of disease occurrence and control in populations.

• Infectious disease: Illness caused by a pathogen (e.g., bacteria, viruses, fungi, protozoa, helminths, prions).

• Epidemiology: The science of monitoring and controlling disease to promote public health.

• Opportunistic pathogens: Cause disease only in weakened hosts.

• True pathogens: Cause disease in healthy hosts.

• Sporadic cases: Isolated infections in a population (e.g., Ebola).

• Endemic infections: Routinely detected in a population or region (e.g., cold viruses).

• Epidemic: Widespread outbreak in a region during a specific time.

• Pandemic: Epidemic that spreads across countries.

• Zoonotic diseases: Spread from animals to humans; many are noncommunicable.

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

• Signs: Objective, measurable indicators (e.g., fever, rash).

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

• Latent infection: Asymptomatic, with no signs or symptoms.

• Acute diseases: Rapid onset and progression.

• Chronic diseases: Slow onset and progression.

Koch’s Postulates

Koch’s postulates are criteria for establishing 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 isolated organism should cause disease in a susceptible host.

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

Limitations include inability to apply to noninfectious diseases, obligate intracellular pathogens, and latent infections.

Reservoirs and Sources of Infection

Pathogens originate from reservoirs (natural habitats) and are transmitted via sources (means of dissemination).

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

• Exogenous source: Pathogen is external to the host.

Modes of Disease Transmission

Pathogens can be transmitted through various routes, including direct and indirect contact, airborne, vehicle, and vector transmission.

• Direct contact: Person-to-person, animal bites, vertical (mother to child).

• Indirect contact: Airborne particles, contaminated objects (fomites), food/water, vectors (biological/mechanical).

Stages of Infectious Disease

Infections progress through five general stages:

• Incubation period: Time between infection and earliest symptoms.

• Prodromal phase: Early, nonspecific symptoms develop.

• 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 illustrates the interaction between the host, etiological agent, and environment in disease development.

• 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.

Public Health Strategies

Strategies to break the epidemiological triangle include public education, quarantine, and vector control.

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 disease unless disrupted (dysbiosis).

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

• Tropism: Pathogen preference for specific host tissues.

• Pathogenicity: Ability to cause disease.

• Virulence: Degree of disease severity.

• Virulence factors: Mechanisms to overcome host defenses (adhesion, invasion, immune evasion, nutrient acquisition, toxin production).

Attenuation

Attenuated pathogens are weakened and often used in vaccines; they have lost some virulence factors but remain infectious.

Toxins as Virulence Factors

Toxins are molecules that damage host tissues or suppress immune responses.

• Toxigenic: Microbes that produce toxins.

• Toxemia: Toxins in the bloodstream.

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

• Exotoxins: Secreted proteins affecting specific cell types (neurotoxins, enterotoxins, etc.); produced by both Gram-positive and Gram-negative bacteria.

Steps to Infection

Pathogens must complete five steps to establish infection:

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

Portals of Entry and Exit

Pathogens enter and exit the body through specific portals (e.g., respiratory mucosa, GI tract, skin, urogenital tract, conjunctiva, parenteral routes).

Cytopathic Effects

Pathogens can cause cytopathic effects by killing or damaging host cells, disrupting normal function, or transforming cells (e.g., cancer).

Immune Evasion Mechanisms

Pathogens evade immune defenses through various strategies:

• Intracellular lifestyle (hiding inside host cells)

• Latency (dormant state)

• Antigenic masking, mimicry, and variation

• Interference with phagocytosis

• Immune suppression (targeting immune cells, degrading antibodies, interfering with signaling)

Innate and Adaptive Immunity

Overview of Immune Responses

The immune system eliminates antigens through innate (nonspecific) and adaptive (specific) responses.

• Innate immunity: Inborn, general defense mechanisms present in all eukaryotes.

• Adaptive immunity: Vertebrate-specific, matures over time, pathogen-specific, exhibits memory.

First-Line Defenses

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

• Mechanical: Flushing, rinsing, trapping (e.g., tears, urine, mucociliary escalator).

• Chemical: Antimicrobial molecules (e.g., lysozyme, stomach acid, fatty acids, antimicrobial peptides).

• Physical: Structural barriers (e.g., skin, mucous membranes).

Lymphoid Tissues and Leukocytes

Primary lymphoid tissues (bone marrow, thymus) produce and mature leukocytes; secondary tissues (lymph nodes, spleen, MALT) filter lymph and sample antigens.

• Leukocytes: White blood cells essential for immune responses; classified as granulocytes (e.g., neutrophils, eosinophils, basophils, mast cells) and agranulocytes (e.g., monocytes, dendritic cells, lymphocytes).

Cytokines and Complement System

Cytokines are signaling proteins that coordinate immune actions. The complement system is a cascade of proteins that enhances phagocytosis, inflammation, and cell lysis.

Inflammation

Inflammation is a key innate response to tissue injury or infection, occurring in three phases: vascular changes, leukocyte recruitment, and resolution.

• Vascular changes: Increased blood flow and vessel permeability.

• Leukocyte recruitment: Neutrophils and monocytes migrate to the site, phagocytize invaders, and recruit more immune cells.

• Resolution: Inflammation subsides, tissue repair begins, and excess leukocytes die off (pus formation).

Fever

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

Immune System Disorders

Immunodeficiencies

Immunodeficiencies are conditions where immune function is impaired.

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

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

Autoimmunity

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

Hypersensitivity Reactions

Hypersensitivities are inappropriate immune responses classified into four types (mnemonic: ACID):

• Type I (Allergy): IgE-mediated, can cause anaphylaxis.

• Type II (Cytotoxic): IgG/IgM-mediated, targets cells (e.g., transfusion reactions, hemolytic disease of the newborn).

• Type III (Immune Complex): Antigen–antibody complexes lodge in tissues (e.g., lupus, serum sickness).

• Type IV (Delayed): T cell-mediated, delayed response (e.g., contact dermatitis, tuberculin skin test, transplant rejection).

Transplantation and Graft Rejection

Transplants are classified by genetic similarity: autografts (self), isografts (identical twin), allografts (same species), xenografts (different species). Graft-versus-host disease may occur in bone marrow transplants.

Immunity Types

Humoral immunity can be acquired naturally (infection, maternal antibodies) or artificially (vaccination, antivenom), and can be active (host makes antibodies) or passive (host receives antibodies).

Visual Summaries

• See images for overviews of immune system organization, inflammation, and hypersensitivity types.