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

Disease Terminology and Epidemiology

Basic Concepts of Infectious Disease

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

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

• Opportunistic pathogens: Cause disease only in weakened hosts.

• True pathogens: Cause disease in healthy hosts.

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

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

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

Patterns of Disease Occurrence

Disease occurrence is classified based on frequency and spread.

• Sporadic: Isolated cases in a population.

• Endemic: Routinely detected in a population or region.

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

• Pandemic: Epidemic that spreads to multiple countries.

Transmission and Disease Stages

Diseases can be communicable or noncommunicable, and their transmission modes are diverse.

• Communicable diseases: Transmit from human to human.

• Contagious diseases: Easily transmitted between hosts.

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

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

• 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 establish the causative relationship between a microbe and a disease.

• 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 applicable to noninfectious diseases.

• Some pathogens cannot be cultured or do not infect nonhuman hosts.

• Latent diseases and obligate intracellular pathogens are exceptions.

Sources and Transmission of Pathogens

Reservoirs and Sources

Pathogens originate from various sources, which can be endogenous (from the host) or exogenous (external).

• Reservoirs: Natural habitats of pathogens.

• Sources: Means by which pathogens are disseminated to new hosts.

Modes of Transmission

Pathogens are transmitted through direct and indirect contact, airborne particles, vehicles (contaminated objects, food, water), and vectors (biological and mechanical).

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

• Indirect contact: Airborne, vehicle, vector transmission.

Stages of Infectious Disease

Five General Stages

Infectious diseases 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 resolve as pathogen replication is controlled.

• Convalescent phase: Recovery; pathogen may remain latent.

Epidemiology Essentials

The Epidemiological Triangle

The epidemiological triangle links the host, etiological agent, and environment, illustrating the factors influencing disease occurrence.

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

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

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

Host–Microbe Interactions and Pathogenesis

Normal Microbiota and Pathogens

Host–microbe interactions are dynamic, with normal microbiota colonizing various body sites and pathogens causing disease.

• Dysbiosis: Disruption of normal microbiota.

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

• Tropism: Pathogen preference for specific host tissues.

Virulence and Virulence Factors

• Pathogenicity: Ability to cause disease.

• Virulence: Degree of disease severity.

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

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 the host through various portals, including respiratory mucosa, skin, ocular, transplacental, parenteral, urogenital, GI mucosa, and otic routes.

Portals of Exit

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

Innate and Adaptive Immunity

Classification of Immune Responses

The immune system is divided into innate and adaptive branches, both recognizing and eliminating pathogens while distinguishing 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 (e.g., tears, urine, saliva, mucus).

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

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

Leukocytes and Lymphoid Tissues

Leukocytes are essential for immune responses and are produced in primary lymphoid tissues (thymus, bone marrow) and filtered in secondary lymphoid tissues (lymph nodes, spleen, MALT).

Complement System and Inflammation

The complement system is a group of proteins that enhance immune responses, leading to opsonization, cytolysis, and inflammation.

Phases of Inflammation

Inflammation is a key innate response, 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.

• Resolution: Healing and reduction of inflammation.

Immune System Disorders

Immunodeficiencies

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

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

Autoimmunity and Hypersensitivities

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

• Hypersensitivities: Inappropriate immune responses classified as allergy (Type I), cytotoxic (Type II), immune complex (Type III), and delayed (Type IV).

Humoral Immunity

Acquisition of Immunity

Humoral immunity can be acquired naturally or artificially, and is either passive (antibodies from another source) or active (antibodies produced by the individual).

• Naturally acquired active immunity: From infection.

• Naturally acquired passive immunity: Antibodies passed from mother to child.

• Artificially acquired active immunity: Vaccination.

• Artificially acquired passive immunity: Antivenom or antibody therapy.