Principles of Infectious Disease, Epidemiology, and Immunity

Disease Terminology and Epidemiology

Basic Concepts in Infectious Disease

Understanding infectious diseases requires familiarity with key terms and principles. Infectious disease is an illness caused by a pathogen, while epidemiology is the study and control of disease occurrence in populations to promote public health.

• Pathogens include prions, viruses, bacteria, protozoans, helminths, and fungi.

• Opportunistic pathogens cause disease mainly in weakened hosts.

• True pathogens can cause disease in healthy hosts.

Patterns of Disease Occurrence

• Sporadic: Isolated infections (e.g., Ebola).

• Endemic: Routinely detected in a population (e.g., cold viruses).

• 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; many are noncommunicable.

Transmission and Disease Progression

• 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 phases.

• Acute diseases: Rapid onset and progression.

• Chronic diseases: Slow onset and progression.

Koch’s Postulates and Their Limitations

Establishing Causation in Infectious Disease

• 1. The same organism must be present in every case of the disease.

• 2. The organism must be isolated and grown as a pure culture.

• 3. The isolated organism should cause disease in a susceptible host.

• 4. The organism must be re-isolated from the inoculated, diseased animal.

Limitations: Not applicable to noninfectious diseases, obligate intracellular pathogens, microbes that lose virulence in culture, or those that do not infect nonhuman hosts.

Sources and Transmission of Pathogens

Reservoirs and Sources

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

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

• Exogenous source: Pathogen is external to the host.

Exogenous Sources	Endogenous Sources
Contaminated food, water, soil, medical equipment; animals; humans	Misplaced normal microbiota; disrupted microbiota and opportunistic pathogens

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 General Stages

Infectious diseases progress through five stages:

• 1. Incubation period: Time between infection and earliest symptoms.

• 2. Prodromal phase: Early, nonspecific symptoms.

• 3. Acute phase: Peak of disease; most severe symptoms.

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

• 5. Convalescent phase: Recovery; pathogen may remain latent.

Epidemiology and the Epidemiological Triangle

Principles of Epidemiology

Epidemiology studies disease patterns in populations to prevent illness. The epidemiological triangle links the host, etiological agent, and environment.

• Host factors: General health, age, sex, lifestyle, genetics.

• Etiological agent: Type of pathogen (bacteria, virus, etc.).

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

Breaking the Triangle

• Public education

• Quarantine

• Vector control

Host–Microbe Interactions and Virulence

Normal Microbiota and Pathogenicity

Normal microbiota colonize various body sites and usually maintain a balanced relationship with the host. Dysbiosis is a disruption of this balance, potentially leading to disease. Tropism refers to a pathogen’s preference for specific hosts or tissues.

• Virulence: Degree of pathogenicity.

• Virulence factors: Mechanisms that help pathogens adhere, invade, evade, and damage host tissues.

Attenuation and Vaccines

Attenuated pathogens are weakened and often used in vaccines because they do not cause disease in healthy hosts but still elicit an immune response.

Toxins as Virulence Factors

Endotoxins vs. Exotoxins

Property	Endotoxins	Exotoxins
Composition	Lipid (LPS)	Protein
Source	Gram-negative bacteria	Gram-negative and Gram-positive bacteria
Release	When bacteria die/divide	Actively secreted
Vaccines	No	Yes (some)
Fever	Yes	Sometimes
Neutralization	No	Yes (some)
Toxicity	Lower (high LD50)	Higher (low LD50)

Types of Exotoxins

• Type I: Membrane-acting extracellular toxins (e.g., superantigens).

• Type II: Membrane-damaging toxins (e.g., hemolysins, cytolysins).

• Type III: Intracellular toxins (e.g., diphtheria, cholera, botulinum toxins).

Steps to Infection

Five Steps 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

Host Immune Evasion and Transmission

Immune Evasion Mechanisms

• Hiding: Antigen masking, mimicry, variation, latency, intracellular lifestyle.

• Undermining: Suppress immune function, avoid phagocytosis, block immune signals.

Transmission and Portals of Exit

Pathogens exit the host via portals such as feces, urine, blood, saliva, mucus, or skin. The portal of entry is often the same as the portal of exit.

Biosafety and Standard Precautions

Biosafety Levels (BSL)

Level	PPE Required	Facility Considerations
BSL-1	None	Hand-washing sinks, open bench, no food/drink
BSL-2	Lab coat, gloves, eye protection	Biohazard signage, autoclave, restricted access
BSL-3	Protective covering, gloves, respirators	Biological safety cabinet, controlled access, special airflow
BSL-4	Full-body suit, air supply	Specialized facility, lockdown access

Standard and Transmission Precautions

• Hand washing, glove changes, barrier clothing, proper waste management, disinfection.

• Transmission precautions: Contact, droplet, and airborne precautions for specific agents.

Innate and Adaptive Immunity

Overview of Immune Responses

The immune system eliminates antigens through innate (nonspecific) and adaptive (specific) responses. Both recognize pathogens, eliminate invaders, and distinguish self from non-self.

• Innate immunity: Inborn, rapid, nonspecific (all eukaryotes).

• Adaptive immunity: Vertebrates only, specific, memory, slower to activate.

First-Line Defenses

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

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

• Physical barriers: Skin, mucous membranes.

Lymphoid Tissues and Leukocytes

• Primary lymphoid tissues: Bone marrow, thymus (leukocyte production/maturation).

• Secondary lymphoid tissues: Lymph nodes, spleen, MALT (antigen sampling).

Leukocytes

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

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

Inflammation and Immune Signaling

Inflammation Phases

• 1. Vascular changes: Increased blood flow and vessel permeability.

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

• 3. Resolution: Healing and tissue repair.

Cytokines

• Chemokines: Recruit leukocytes.

• Interleukins: Regulate inflammation, fever, immune responses.

• Interferons: Antiviral responses.

• Tumor necrosis factors: Inflammation, kill tumor cells.

Immune System Disorders

Immunodeficiencies

• Primary (congenital): Genetic defects (e.g., SCID, DiGeorge syndrome).

• Secondary (acquired): Aging, infections, medical interventions, systemic disorders.

Autoimmunity

• Immune attack against self-tissues (e.g., lupus, type I diabetes, rheumatoid arthritis).

• Symptoms: Joint/muscle pain, fatigue, rash, organ dysfunction.

Hypersensitivity Reactions

Type	Description	Examples
I (Allergy)	IgE-mediated, immediate	Allergies, anaphylaxis
II (Cytotoxic)	IgG/IgM to cell-bound antigens	Hemolytic disease of newborn, transfusion reactions
III (Immune complex)	IgG/IgM to soluble antigens	Lupus, rheumatoid arthritis
IV (Delayed)	T cell-mediated	Contact dermatitis, graft rejection

Blood Typing and Transfusion Compatibility

Blood Type	Antigens Present	Can Receive From
AB+	A, B, Rh	All types (universal recipient)
O-	None	O- only (universal donor)

Immunity Acquisition

• Naturally acquired active: Infection-induced immunity.

• Naturally acquired passive: Maternal antibodies.

• Artificially acquired active: Vaccination.

• Artificially acquired passive: Antivenom, antibody therapy.

Additional info: This guide integrates foundational concepts from microbiology chapters on infectious disease, epidemiology, host–microbe interactions, immunity, and immune disorders, with tables and images to reinforce key points.