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

Disease Terminology and Epidemiology

Basic Principles of Infectious Disease and Epidemiology

Infectious diseases are illnesses caused by pathogens, and epidemiology is the study of disease occurrence and control to promote public health. Understanding these concepts is fundamental for identifying, tracking, and preventing disease outbreaks.

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

• Opportunistic pathogens: Cause disease only in weakened hosts.

• True pathogens: Cause disease in any susceptible host.

• Sporadic cases: Isolated infections in a population.

• Endemic infections: Routinely detected in a population or region.

• 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.

Emerging and Reemerging Pathogens

Emerging pathogens are newly identified or have expanded distribution, while reemerging pathogens were previously controlled but are resurfacing. These agents are likely to cause epidemics or pandemics.

• Examples: SARS-CoV-2 (emerging), antibiotic-resistant bacteria (reemerging).

Communicable vs. Noncommunicable Diseases

• Communicable diseases: Transmit from human to human.

• Contagious diseases: Easily transmitted between hosts.

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

• Acute diseases: Rapid onset and progression.

• Chronic diseases: Slow onset and progression.

Koch’s Postulates and Limitations

Identifying Causative Agents

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

• 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 inoculated 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

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

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

• Exogenous source: Pathogen is external to the host.

Modes of Transmission

Pathogens can be transmitted through various modes, including direct contact, airborne, vehicle, vector (biological and mechanical), and vertical transmission.

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

• Indirect contact: Airborne particles, contaminated objects, food, water, vectors.

Stages of Infectious Disease

Five General Stages

Infectious diseases progress through five stages: incubation, prodromal, acute, period of decline, and convalescent phase.

• 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.

Epidemiology Essentials

The Epidemiological Triangle

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

• 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.

Host–Microbe Interactions and Pathogenesis

Normal Microbiota and Pathogens

Host–microbe interactions are dynamic. Normal microbiota colonize various body sites and usually do not cause disease unless disrupted (dysbiosis). Pathogens have adaptations to interact with host tissues and may cause disease.

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

• Tropism: Pathogen preference for specific host tissues.

Virulence and Virulence Factors

• Pathogenicity: Ability of a microbe to cause disease.

• Virulence: Degree or extent of disease caused.

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

Steps to Infection

Five Steps to Establish Infection

Pathogens must complete five tasks: enter the host, adhere to tissues, invade and obtain nutrients, replicate while evading immune defenses, and transmit to a new host.

• Portals of entry: Skin, respiratory mucosa, GI mucosa, urogenital, ocular, otic, parenteral, transplacental.

Host Immune Defenses and Evasion

Immune Evasion Mechanisms

Pathogens evade immune defenses through various strategies:

• Intracellular lifestyle: Pathogens reside inside host cells.

• Latency: Pathogen exists quietly, causing persistent or recurrent disease.

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

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

• Immune suppression: Targeting immune cells, breaking down antibodies, interfering with signaling.

Transmission and Portals of Exit

Transmission to New Hosts

Pathogens exit the host via portals of exit, often the same as the portal of entry. Symptoms such as sneezing, coughing, diarrhea, and itchiness facilitate transmission.

• Portals of exit: Feces, urine, blood, wound drainage, saliva, mucus, semen.

Innate and Adaptive Immunity

Classification of Immune Responses

The immune system has two main branches: innate and adaptive immunity. Both recognize pathogens, eliminate invaders, and distinguish self from foreign antigens.

• Innate immunity: Inborn, nonspecific, rapid response.

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

First-Line Defenses

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

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

• Chemical barriers: Molecules that attack microbes or create hostile environments (lysozyme, stomach acid, fatty acids).

• Physical barriers: Structures that block entry (skin, mucous membranes).

Leukocytes and Lymphoid Tissues

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

Inflammation and Immune Response

Inflammation Phases

Inflammation is a key innate immune response, essential for healing. It occurs in three phases: vascular changes, leukocyte recruitment, and resolution.

• Vascular changes: Blood flow and vessel permeability increase.

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

• Resolution: Inflammation signals decrease, tissue repair begins.

Immune System Disorders

Primary and Secondary Immunodeficiencies

• Primary immunodeficiency: Congenital, genetic defects affecting immune function.

• Secondary immunodeficiency: Acquired, due to aging, chronic disease, infections, or medical interventions.

Autoimmunity and Hypersensitivity

• Autoimmunity: Immune system attacks self-tissues, leading to chronic disorders (e.g., lupus, rheumatoid arthritis).

• Hypersensitivity: Inappropriate immune responses, classified into four types (ACID mnemonic: Allergy, Cytotoxic, Immune Complex, Delayed).

Humoral Immunity

Acquisition of Immunity

Humoral immunity can be acquired naturally or artificially, and is either passive or active.

• Active immunity: Individual produces antibodies (infection or vaccination).

• Passive immunity: Individual receives antibodies (placental transfer, antivenom).

Visual Summary: Innate and Adaptive Immunity