Infectious Disease, Epidemiology, and Immune System Study Notes

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Disease Terminology and Epidemiology

Basic Terms and Principles

Understanding infectious diseases and their spread is essential for public health. Infectious disease is an illness caused by a pathogen, while epidemiology is the study of disease occurrence and control in populations.

Pathogens: Prions, viruses, bacteria, protozoans, helminths, fungi
Opportunistic pathogens: Cause disease in weakened hosts
True pathogens: Cause disease in healthy hosts

Disease Occurrence Patterns

Sporadic: Isolated cases (e.g., Ebola)
Endemic: Routinely detected in a population (e.g., cold viruses)
Epidemic: Widespread outbreak in a region
Pandemic: Epidemic that spreads globally

Emerging, Reemerging, and Zoonotic Diseases

Emerging pathogens: Newly identified or expanding (e.g., SARS-CoV-2)
Reemerging pathogens: Previously controlled, now resurfacing (e.g., antibiotic-resistant bacteria)
Zoonotic diseases: Spread from animals to humans

Communicable vs. Noncommunicable Diseases

Communicable: Transmit human to human
Contagious: Easily transmitted
Noncommunicable: Not spread person to person

Signs, Symptoms, and Disease Duration

Signs: Objective, measurable (e.g., fever, rash)
Symptoms: Subjective, felt by patient (e.g., pain, fatigue)
Acute: Rapid onset/progression
Chronic: Slow onset/progression

Koch’s Postulates and Their Limitations

Koch’s Postulates

Same organism present in every case
Organism isolated and grown in pure culture
Isolated organism causes disease in susceptible host
Organism re-isolated from 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

Reservoir: Natural habitat of pathogen
Source: Where the host acquires the pathogen
Endogenous: From host’s own body
Exogenous: From external environment

Modes of Transmission

Direct contact: Person-to-person, animal bites, vertical (mother to child)
Indirect contact: Airborne, vehicle (fomites, food, water), vector (biological/mechanical)

Stages of Infectious Disease

Five Stages of Disease

Infectious diseases progress through five general stages:

1. Incubation period: Time between infection and earliest symptoms
2. Prodromal phase: Early, mild symptoms
3. Acute phase: Peak of disease
4. Period of decline: Symptoms resolve as pathogen is controlled
5. Convalescent phase: Recovery; pathogen may remain latent

Graph of the five stages of infectious disease

Epidemiological Triangle

Host, Agent, and Environment

The epidemiological triangle illustrates the interaction between the host, the etiological agent, and the environment in disease development.

Host factors: Health, age, lifestyle, genetics
Agent: Type of pathogen (bacteria, virus, etc.)
Environmental factors: Climate, geography, vectors, water/food sources

Epidemiological triangle diagram

Prevention Strategies

Public education
Quarantine
Vector control

Host–Microbe Interactions and Virulence

Normal Microbiota and Pathogenicity

Normal microbiota: Colonize skin, digestive, genital, urinary, and respiratory systems
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: Severity of disease caused
Virulence factors: Mechanisms to overcome host defenses (adhesion, invasion, immune evasion, nutrient acquisition, toxins)

Diagram of bacterial virulence factors

Attenuation

Attenuated pathogens: Weakened, often used in vaccines

Toxins as Virulence Factors

Types of Toxins

Endotoxins: Part of Gram-negative cell wall (lipopolysaccharide, LPS); released upon cell death; can cause septic shock
Exotoxins: Secreted proteins; affect various tissues (neurotoxins, enterotoxins, etc.); produced by both Gram-positive and Gram-negative bacteria

Steps to Infection

Five Steps to Successful 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 and Exit

Pathogens use specific portals to enter and exit the host, often the same for both processes.

Diagram of portals of entry for pathogens Diagram of portals of exit for pathogens

Immune System Overview

Innate vs. Adaptive Immunity

Innate immunity: Inborn, nonspecific, immediate response
Adaptive immunity: Specific, develops over time, has memory

Diagram of three lines of immune defense

First-Line Defenses

Mechanical barriers: Flushing, rinsing, trapping (e.g., tears, urine, mucus)
Chemical barriers: Molecules that attack microbes (e.g., lysozyme, stomach acid, antimicrobial peptides)
Physical barriers: Structural blockades (e.g., skin, mucous membranes)

Diagram of first-line immune defenses

Lymphoid Tissues and Leukocytes

Primary lymphoid tissues: Thymus, bone marrow (leukocyte production/maturation)
Secondary lymphoid tissues: Lymph nodes, spleen, MALT (filter lymph, sample antigens)
Leukocytes: White blood cells essential for immune responses

Types of leukocytes

Cytokines and Complement System

Cytokines: Signaling proteins for immune cell communication
Complement system: Protein cascade that enhances phagocytosis, inflammation, and cell lysis

Complement cascade and outcomes