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

Summary Table: Immune System Disorders