Microbiology Study Guide: Infectious Disease, Pathogenesis, and Immunity

Disease Terminology and Epidemiology

Basic Concepts in Infectious Disease and Epidemiology

Understanding infectious disease requires knowledge of key terms and principles. Infectious disease is an illness caused by a pathogen, while epidemiology is the study of disease occurrence and control 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.

Patterns of Disease Occurrence

Diseases can be classified based on their frequency and spread:

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

• Endemic: 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.

Emerging, Reemerging, and Zoonotic Diseases

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

• Reemerging pathogens: Previously controlled agents resurfacing (e.g., antibiotic-resistant bacteria).

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

Communicable vs. Noncommunicable Diseases

• Communicable diseases: Transmit from human to human.

• Contagious diseases: Easily transmitted between hosts.

• Active infection: Patient is symptomatic (signs and symptoms).

• Latent infection: Usually asymptomatic.

Onset and Duration of Disease

• Acute diseases: Rapid onset and progression.

• Chronic diseases: Slower onset and progression.

Koch’s Postulates and Their Limitations

Koch’s Postulates

Koch’s postulates are a set of criteria used to identify the causative agent of infectious diseases:

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

2. The organism must be isolated and grown as a pure culture.

3. The isolated organism should cause disease when inoculated into a susceptible host.

4. The organism must be re-isolated from the inoculated, diseased animal.

Limitations

• Do not apply to noninfectious diseases.

• Some infectious agents cannot be isolated or grown in the lab.

• Some microbes become attenuated in pure cultures.

• Some microbes do not infect nonhuman hosts.

• Not easily applied to latent diseases.

Sources and Transmission of Pathogens

Reservoirs and Sources

Reservoirs are habitats where pathogens are naturally found. Sources disseminate infectious agents from reservoirs to new hosts.

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

• Exogenous source: Pathogen is external to the host.

Exogenous Sources	Endogenous Sources
Contaminated food, medical equipment, soil, water	Misplaced normal microbiota (e.g., skin bacteria causing surgical infections)
Animals (zoonotic diseases)	Disrupted microbiota and opportunistic pathogens (e.g., yeast infection after antibiotics)
Humans (communicable infections)

Modes of Transmission

Pathogens can be transmitted through various routes:

• Direct contact: Skin/mucous membrane contact, bodily fluids, vertical transmission (mother to child).

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

Stages of Infectious Disease

Five General Stages

Infections progress through five stages:

• Incubation period: Time between infection and earliest symptoms.

• Prodromal phase: Early symptoms develop.

• Acute phase: Peak of disease.

• Period of decline: Symptoms start to resolve.

• Convalescent phase: Recovery; pathogen may remain latent.

Epidemiological Triangle

Host, Etiological Agent, and Environment

The epidemiological triangle links three factors: host, etiological agent, and environment. Disease occurs when these factors interact.

• Host factors: General health, sex, lifestyle, age, ethnicity, occupation.

• Etiological agent: Fungi, bacteria, virus, parasite, or prion.

• Environmental factors: Climate, geographic location, vector availability, water/food sources.

Host–Microbe Interactions and Pathogenesis

Normal Microbiota and Pathogens

Host–microbe interactions are dynamic. Normal microbiota colonize various body sites and usually coexist with the immune system. Dysbiosis is disruption of this balance.

• 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 caused.

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

Virulence Factors

Virulence factors include toxins, enzymes, and structures that help pathogens adhere, invade, and evade immune responses.

Toxins and Their Role in Disease

Endotoxins vs. Exotoxins

Toxins are major virulence factors. Endotoxins are lipid molecules from Gram-negative bacteria, released when bacteria die. Exotoxins are soluble proteins made by both Gram-positive and Gram-negative bacteria.

Property	Endotoxins	Exotoxins
Made of	Lipid	Protein
Made by	Gram-negative bacteria	Gram-negative and Gram-positive bacteria
Released from	Cell wall when bacteria divide or die	Actively growing bacteria
Vaccines	No	Yes (some)
Fever	Yes	Sometimes
Can be neutralized	No	Yes (some)
Toxicity level	Lower (relatively high LD50)	Higher (low LD50)

Exotoxin Types and Examples

• Type I: Membrane-acting extracellular toxins (e.g., superantigens, enterotoxins).

• Type II: Membrane-damaging toxins (e.g., hemolysins, cytolysins, phospholipases).

• Type III: Intracellular toxins (e.g., diphtheria toxin, cholera toxin, botulinum toxin).

Steps to Infection and Portals of Entry/Exit

Five Steps to Infection

Pathogens must complete five tasks 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

Pathogens enter through specific portals: respiratory mucosa, skin, ocular, transplacental, parenteral, urogenital, GI mucosa, otic, urinary.

Portals of Exit

Pathogens exit the host via feces, urine, blood, wound drainage, saliva, mucus, semen, etc. The portal of entry is often the same as the portal of exit.

Immune System and Host Defense

Innate and Adaptive Immunity

The immune system has two main branches: innate immunity (inborn, nonspecific) and adaptive immunity (specific, develops over time).

• Both recognize diverse pathogens, eliminate invaders, and discriminate self from foreign antigens.

• Adaptive immunity exhibits memory and is only found in vertebrates.

First-Line Defenses

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

• Mechanical: Flushing, rinsing, trapping (e.g., tears, urine, saliva, mucus membranes).

• Chemical: Molecules that attack microbes or limit survival (e.g., lysozyme, stomach acid, fatty acids).

• Physical: Structural blockade (e.g., skin, mucociliary escalator).

Primary and Secondary Lymphoid Tissues

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

Leukocytes and Their Roles

Leukocytes are essential for immune responses. They are classified as granulocytes (neutrophils, eosinophils, basophils, mast cells) and agranulocytes (monocytes, dendritic cells, lymphocytes).

Inflammation and Immune Response

Phases of Inflammation

Inflammation is a key innate immune response, essential for healing. It occurs in three phases:

• Vascular changes: Blood flow and vessel permeability increase.

• Leukocyte recruitment: Immune cells are recruited to the site.

• Resolution: Inflammation decreases, tissue repair begins.

Fever as an Immune Response

Fever enhances antiviral effects, increases phagocyte efficiency, limits pathogen growth, and promotes tissue repair.

Immune System Disorders

Primary and Secondary Immunodeficiencies

Primary immunodeficiencies are congenital and affect immune factors. Secondary immunodeficiencies are acquired and more common, caused by aging, disease, or medical interventions.

Autoimmunity and Hypersensitivity

Autoimmunity is an immune attack against self-tissues. Hypersensitivities are inappropriate immune responses, classified as:

Class	Description	Examples
Type I	Allergy (IgE-mediated)	Hives, anaphylaxis
Type II	Cytotoxic (IgG/IgM-mediated)	Hemolytic disease of the newborn, transfusion reactions
Type III	Immune complex (IgG/IgM-mediated)	Lupus, rheumatoid arthritis
Type IV	Delayed (T cell-mediated)	Contact dermatitis, transplant rejection

Summary: Host–Microbe Interactions and Pathogenesis

Summary: Innate and Adaptive Immunity