Microbiology Study Guide: Infectious Disease, Epidemiology, and Immunity (Chapters 9–13)

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 is the study and control of disease occurrence in populations 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.

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

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

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

• Pandemic: Epidemic that spreads to multiple 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 phase.

• Acute diseases: Rapid onset and progression.

• Chronic diseases: Slow onset and progression.

Koch’s Postulates and Their Limitations

Koch’s postulates are a set of criteria used to establish a causative relationship between a microbe and a disease. However, they have limitations, especially for noninfectious diseases, obligate intracellular pathogens, and latent infections.

• The same organism must be present in every case.

• 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 diseased host.

Limitations: Not all pathogens can be cultured, some only infect humans, and some cause latent or asymptomatic infections.

Sources and Transmission of Pathogens

Pathogens originate from various sources, which can be endogenous (from the host) or exogenous (external). Reservoirs are habitats where pathogens are naturally found.

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

• Indirect contact transmission: Airborne, vehicle (fomites, food, water), vector (biological/mechanical).

Stages of Infectious Disease

Infections progress through five general stages: incubation, prodromal, acute, decline, and convalescent phases. Infectivity, pathogenicity, and virulence describe how well a pathogen establishes infection, causes disease, and the severity of disease, respectively.

• Incubation period: Time between infection and earliest symptoms.

• Prodromal phase: Early, mild symptoms.

• Acute phase: Peak of disease.

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

• Convalescent phase: Recovery; pathogen may remain latent.

Epidemiology and Public Health

The Epidemiological Triangle

The epidemiological triangle links the host, etiological agent, and environment. Disease prevention strategies target one or more of these components.

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

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

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

Public Health Strategies

• Public education

• Quarantine

• 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). Pathogens have adaptations for host interaction and tissue tropism.

• 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).

Five Steps to Infection

To establish infection, pathogens 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

Portals of Entry and Exit

Pathogens use specific portals to enter and exit the host, often the same anatomical site. Examples include respiratory mucosa, skin, GI tract, urogenital tract, and parenteral routes.

Host Immune Evasion

Pathogens evade immune defenses by hiding (intracellular lifestyle, latency, antigenic masking/mimicry/variation) and undermining immune responses (inhibiting phagocytosis, immune suppression).

Toxins and Virulence Factors

Types of Toxins

Toxins are major virulence factors. Endotoxins (LPS from Gram-negative bacteria) can cause septic shock. Exotoxins are secreted proteins with diverse targets (neurotoxins, enterotoxins, etc.).

• Toxigenic: Microbes that produce toxins.

• Toxemia: Toxins in the bloodstream.

Innate and Adaptive Immunity

Overview of Immune Responses

The immune system eliminates antigens through innate and adaptive branches. 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).

• 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.

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 and Its Phases

Inflammation is a key innate response with three phases: vascular changes, leukocyte recruitment, and resolution. It recruits immune cells, limits pathogen spread, and promotes healing.

• Vascular changes: Increased blood flow and vessel permeability.

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

• Resolution: Inflammation subsides, tissue repair begins.

Fever

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

Immune System Disorders

Immunodeficiencies

Immunodeficiencies can be primary (genetic) or secondary (acquired). Primary immunodeficiencies affect immune factors from birth; secondary are due to aging, infections, or medical interventions.

Autoimmunity and Hypersensitivities

Autoimmunity is an immune attack against self-tissues, leading to chronic disorders. Hypersensitivities are inappropriate immune responses classified into four types (ACID mnemonic):

• Type I (Allergy): IgE-mediated, rapid (e.g., anaphylaxis).

• Type II (Cytotoxic): IgG/IgM-mediated, cell lysis (e.g., transfusion reactions).

• Type III (Immune Complex): Antigen–antibody complexes (e.g., lupus).

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

Transplantation and Graft Rejection

Transplants are classified as autografts, isografts, allografts, or xenografts. Graft-versus-host disease may occur in bone marrow transplants when donor immune cells attack the recipient.

Blood Typing and Rh Incompatibility

Blood typing prevents transfusion reactions. Rh incompatibility during pregnancy can lead to hemolytic disease of the newborn if maternal antibodies attack fetal red blood cells.

Allergy Diagnosis and Management

Allergies are diagnosed by blood tests (IgE titers) or skin tests (wheal and flare). Management may include desensitization therapy and avoidance of allergens.