Microbiology Study Guide: Infectious Disease, Epidemiology, Host-Microbe Interactions, and Immunity

Disease Terminology and Epidemiology

Basic Concepts in Infectious Disease

Infectious diseases are caused by pathogens and are a central focus of microbiology and public health. Epidemiology is the study and control of disease occurrence in populations.

• Pathogens: Prions, viruses, bacteria, protozoans, helminths, and fungi.

• Opportunistic pathogens: Cause disease only in weakened hosts.

• True pathogens: Cause disease in healthy hosts.

Patterns of Disease Occurrence

Understanding how diseases spread and persist is crucial for epidemiology.

• Sporadic: Isolated cases in a population.

• Endemic: Routinely detected in a population or region.

• Epidemic: Widespread outbreak in a region.

• Pandemic: Epidemic that spreads across countries.

Emerging, Reemerging, and Zoonotic Diseases

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

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

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

Communicable vs. Noncommunicable Diseases

• Communicable: Transmit from human to human.

• Contagious: Easily transmitted between hosts.

• Latent infection: Asymptomatic, no signs or symptoms.

Acute vs. Chronic Diseases

• Acute: Rapid onset and progression.

• Chronic: Slow onset and progression.

Koch’s Postulates and Their Limitations

Establishing Causation in Infectious Disease

Koch’s postulates are foundational for identifying causative agents of infectious diseases.

• The same organism must be present in every case.

• Organism must be isolated and grown in pure culture.

• Isolated organism should cause disease in a susceptible host.

• Organism must be re-isolated from the diseased host.

Limitations

• Not applicable to noninfectious diseases.

• Some pathogens cannot be isolated or grown in lab.

• Obligate intracellular pathogens and latent diseases are exceptions.

Sources and Transmission of Pathogens

Reservoirs and Sources

Pathogens originate from various sources, which are critical for understanding transmission.

• Reservoirs: Natural habitats (animate or inanimate).

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

• Exogenous source: Pathogen external to host.

Modes of Transmission

Pathogens can be transmitted through direct or indirect contact, environmental sources, vectors, and vertical transmission.

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

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

Example: Transmission Modes

Stages of Infectious Disease

Five General Stages

Infectious diseases progress through distinct stages, each with characteristic pathogen levels and symptoms.

• Incubation period: Time between infection and earliest symptoms.

• Prodromal phase: Early symptoms develop.

• Acute phase: Peak of disease.

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

• Convalescent phase: Recovery; pathogen may remain latent.

Epidemiology Essentials

The Epidemiological Triangle

Disease occurrence is influenced by the interaction of host, etiological agent, and environment.

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

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

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

Public Health Strategies

• Education, quarantine, and vector control are key strategies to break the triangle and prevent disease.

Host–Microbe Interactions and Pathogenesis

Normal Microbiota and Pathogens

Host–microbe interactions are dynamic; normal microbiota colonize various body sites and can be disrupted (dysbiosis). Pathogens have adaptations for tissue interaction and can cause disease.

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

• Tropism: Pathogen preference for specific host tissues.

Pathogenicity and Virulence

• Pathogenicity: Ability to cause disease.

• Virulence: Degree of disease caused.

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

Example: Virulence Factors

Attenuation

• Attenuated pathogens: Weakened, often used in vaccines.

Toxins as Virulence Factors

Types of Toxins

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

• Exotoxins: Soluble proteins affecting various cell types (neurotoxins, enterotoxins, hepatotoxins, nephrotoxins).

Mechanisms of Exotoxin Action

• Type I: Bind to host membrane, signal without entering cell.

• Type II: Disrupt host cell membranes (pore formation, lipid hydrolysis).

• Type III: Enter cell via endocytosis, active portion exerts effect.

Steps to Infection and Immune Evasion

Five Steps to Infection

Successful 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 matching transmission mode.

Immune Evasion Mechanisms

• Intracellular lifestyle: Pathogens hide inside host cells.

• Latency: Pathogen remains dormant.

• Antigenic masking, mimicry, variation: Conceal or alter antigens to avoid immune detection.

• Interference with phagocytosis: Capsules, blocking phagosome-lysosome fusion, neutralizing enzymes.

• Immune suppression: Target immune cells, break down antibodies, interfere with signaling.

Innate and Adaptive Immunity

Overview of Immune Responses

The immune system eliminates antigens through innate and adaptive branches.

• Innate immunity: Inborn, nonspecific, rapid response.

• Adaptive immunity: Specific, develops over time, memory, slower to activate.

First-Line Defenses

• Mechanical barriers: Flushing, rinsing, trapping (tears, urine, saliva, mucus, mucociliary escalator).

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

• Physical barriers: Skin, mucous membranes.

Lymphoid Tissues and Leukocytes

• Primary lymphoid tissues: Thymus, bone marrow (leukocyte production and maturation).

• Secondary lymphoid tissues: Lymph nodes, spleen, MALT (filter lymph, sample antigens).

Leukocyte Types

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

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

Cytokines and Complement System

• Cytokines: Signaling proteins for immune communication; can cause cytokine storm.

• Complement cascades: Series of proteins leading to opsonization, cytolysis, and inflammation.

Inflammation

• Three phases: Vascular changes, leukocyte recruitment, resolution.

• Goals: Recruit defenses, limit spread, promote healing.

Fever

• Systemic response; enhances immune efficiency and tissue repair.

Immune System Disorders

Primary and Secondary Immunodeficiencies

• Primary: Genetic defects affecting immune factors.

• Secondary: Acquired, due to age, infection, medical intervention, or systemic disorders.

Autoimmunity and Hypersensitivities

• Autoimmunity: Immune attack against self-tissues.

• Hypersensitivities: Inappropriate immune responses; classified as Type I-IV (ACID mnemonic).

Humoral Immunity

Acquisition of Immunity

• Natural: From infection or maternal antibodies.

• Artificial: From vaccination or antivenom.

• Active: Individual produces antibodies.

• Passive: Individual receives antibodies.

Summary Table: Disease Transmission Modes

Mode	Example Pathogens	Transmission Route
Direct contact	HIV, chlamydia, MRSA	Person-to-person, bodily fluids
Indirect contact	Salmonella, cholera	Food, water, fomites
Vector (biological)	Malaria, Lyme disease	Mosquito, tick bites
Vector (mechanical)	Plague	Flea bite
Vertical	HIV, syphilis	Mother to child

Summary Table: Immune System Disorders

Disorder Type	Examples	Key Features
Primary immunodeficiency	SCID, DiGeorge syndrome	Genetic, affects immune factors
Secondary immunodeficiency	HIV, malnutrition	Acquired, affects normal immune system
Autoimmunity	Lupus, rheumatoid arthritis	Immune attack against self
Hypersensitivity	Allergy, transfusion reaction	Inappropriate immune response