Chapter 10 (05/26) — Mechanisms of Infectious Disease Chapter 10 — Mechanisms of Infectious Disease
BIG IDEA 🦠
Infectious disease happens when a microorganism enters the body, survives, multiplies, and causes damage.
Infection depends on 3 things:
The infectious agent
A susceptible host
An environment that supports infection
KEY TERMS
Host
A living organism that supports another organism’s growth.
Example: A human body can be a host for bacteria, viruses, fungi, or parasites.
Infectious Disease
A disease caused by interaction with another organism.
The host becomes injured or damaged.
Colonization
Microorganisms are present and multiplying on or inside the body.
Important: Colonization does NOT always mean disease.
Microflora
Normal bacteria living on exposed body surfaces.
Example:
Skin bacteria
Gut bacteria
Vaginal flora
Virulence
The ability of a microorganism to cause disease.
High virulence = more likely to cause serious disease.
PATHOGENS
Pathogens are microorganisms that are very likely to cause disease.
They are usually not found without illness.
Examples:
Influenza virus
Strep bacteria
Candida fungus
Malaria parasite
SAPROPHYTES
Free-living organisms that grow on dead or decaying organic material.
They usually live in the environment.
TYPES OF HOST-MICROBE RELATIONSHIPS
Mutualism
Both the host and microorganism benefit.
Example:
Gut bacteria help digest food.
The bacteria get nutrients and shelter.
Commensalism
The microorganism benefits.
The host is not helped or harmed.
Example:
Some skin bacteria live on the skin without causing harm.
Parasitic Relationship
Only the microorganism benefits.
The host is harmed.
Example:
A parasite takes nutrients from the host and causes disease.
INFECTIOUS DISEASE CONCEPT
Infection occurs when:
A pathogen is present
The host is susceptible
The environment allows the pathogen to survive/spread
Simple way to remember:
Agent + Host + Environment = Infection Risk
COMMON AGENTS OF INFECTIOUS DISEASE
Viruses
Made of DNA or RNA inside a protein coat.
Cannot reproduce outside cells.
Must use the host’s cells to replicate.
Examples:
Influenza
Common cold
Measles
HIV
SARS-CoV-2
Treatment:
Antivirals
Antivirals usually slow viral replication.
Bacteria
Microscopic cells without a nucleus.
Can reproduce on their own.
Found on many surfaces.
Examples:
Strep throat
Sinus infections
Pneumonia
Food poisoning
Treatment:
Antibiotics
Antibiotics slow bacterial growth or kill bacteria.
Fungi
Include yeasts and molds.
Often infect body surfaces or body openings.
Examples:
Athlete’s foot
Candida yeast infections
Treatment:
Antifungals
Many antifungals target fungal cell walls or membranes.
Protozoa
Microscopic single-celled organisms.
Often found in contaminated water or transmitted by vectors.
Examples:
Malaria
Sleeping sickness
Treatment:
Antiprotozoals
These interfere with protozoa metabolism.
Worms / Helminths
Multicellular parasites.
Can live in body spaces or tissues.
Examples:
Roundworm
Tapeworm
Flukes
Treatment:
Antihelmintics
These interfere with worm metabolism.
Prions
Misfolded proteins.
Not bacteria, viruses, fungi, or parasites.
Cause serious neurodegenerative disease.
Examples:
Creutzfeldt-Jakob disease
Kuru
Mad cow disease
Treatment:
Still being researched.
Prions are very resistant to enzymes and treatment.
PRIONS
What They Are
Prions are abnormal misfolded proteins.
Normal protein = PrPC
Misfolded infectious protein = PrPSC
What They Do
Build up in nervous system tissue.
Damage neurons.
Cause brain degeneration.
Effects
Ataxia = poor coordination
Dementia
Death
Important
Prions are resistant to many treatments.
Accumulation is toxic to cells.
VIRUSES
Main Facts
Smallest obligate intracellular pathogen.
“Obligate intracellular” means they MUST live inside a host cell to reproduce.
Structure
Genetic material:
DNA or RNA
Protein coat:
Capsid
Some viruses also have:
Envelope
Viruses Are Classified By
Genome type
Physical characteristics
Replication method
Transmission method
Type of disease
Tissue they infect
How Viruses Affect Host Cells
Viruses can:
Kill the host cell by lysis
Stay latent in the cell
Actively replicate and cause disease
Transform the host cell into a cancerous/malignant cell
Latent Virus
Virus hides in the body without actively causing symptoms.
Can reactivate later.
Example:
Herpes virus can become latent and return later.
BACTERIA
Main Facts
Bacteria are prokaryotes.
Prokaryote = cell without a nucleus.
They have a rigid cell wall.
They reproduce by cellular division.
Bacteria Are Grouped By
Shape
Cocci = round
Bacilli = rod-shaped
Spirilla = spiral-shaped
Gram Stain
Gram-positive
Gram-negative
Oxygen Use
Aerobes = need oxygen
Anaerobes = do not need oxygen
RICKETTSIA, ANAPLASMA, CHLAMYDIA, COXIELLA
These organisms have features of both viruses and bacteria.
Virus-like Features
They are obligate intracellular pathogens.
This means they must live inside cells.
Bacteria-like Features
Have rigid peptidoglycan cell walls.
Reproduce by cellular division.
Contain RNA and DNA.
PARASITES
Protozoa
Single-celled animals.
Transmission
Person-to-person contact
Sexual transmission
Contaminated water
Contaminated food
Vectors such as mosquitoes
Helminths
Worms.
Examples:
Tapeworms
Roundworms
Flukes
Transmission
Ingesting fertilized eggs
Penetration through skin
Arthropods
Small organisms such as:
Mites
Lice
Fleas
EPIDEMIOLOGY TERMS
Epidemiology
Study of how infectious diseases spread in humans.
It looks at:
Factors
Events
Circumstances
Transmission patterns
Incidence
Number of NEW cases in a specific population during a specific time.
Simple:
New cases
Prevalence
Number of ACTIVE cases at a specific time.
Simple:
Total current cases
INCIDENCE OF DISEASE
Endemic Disease
Disease regularly found in a specific geographic area.
Incidence and prevalence are expected and stable.
Example:
Malaria is endemic in some regions.
Epidemic
Sudden unexpected increase in disease above normal levels.
Simple:
More cases than expected in one area.
Pandemic
Disease spreads beyond continental boundaries.
Simple:
Worldwide or multi-continent spread.
MODES OF TRANSMISSION
Transmission = how a pathogen spreads.
Portal of Entry
How the pathogen enters the body.
The pathogen must enter the body, reach tissues, and cause disease.
1. Penetration
Pathogen enters through broken skin or damaged mucous membranes.
Examples:
Cuts
Needlestick injuries
Open wounds
2. Direct Contact
Pathogen moves directly from infected tissue or secretions to another person.
Examples:
Touching infected fluid
Sexual contact
STIs
3. Ingestion
Pathogen enters through the mouth and GI tract.
Examples:
Hepatitis A
Food poisoning
Contaminated food/water
4. Inhalation
Pathogen enters through the respiratory tract.
Examples:
Chickenpox
Pneumonia
Common cold
Flu
Tuberculosis
COVID-19
MECHANISMS OF DISEASE PRODUCTION
Microbes cause disease by damaging tissue.
They can:
Enter host cells and directly kill them
Trigger immune responses that damage host tissue
Produce virulence factors
VIRULENCE FACTORS
Virulence factors are substances or traits that help microbes cause disease.
Main types:
Toxins
Adhesion factors
Evasion factors
Invasion factors
TOXINS
Toxins alter or destroy normal host cell function.
Endotoxins
Found in the cell wall of gram-negative bacteria.
Made of lipopolysaccharides, or LPS.
Activate inflammation.
Simple:
Endotoxins are part of the bacteria’s outer structure.
When released, they can cause strong inflammation.
Exotoxins
Proteins released by bacteria during growth.
Damage host cells.
Can cause cell dysfunction or death.
Simple:
Exotoxins are poisons bacteria release.
Enterotoxins
A type of exotoxin that affects the GI tract.
Causes:
Vomiting
Diarrhea
ADHESION FACTORS
Help microbes attach to the host.
If a microbe cannot attach, it may be washed away.
Simple:
Adhesion factors help germs “stick.”
EVASION FACTORS
Help microbes avoid the immune system.
Example:
Encapsulation
Encapsulation
Some bacteria have a capsule.
The capsule makes it harder for immune cells to recognize and destroy them.
Simple:
Capsule = protective shield.
INVASION FACTORS
Help microbes enter tissues and spread.
Often involve enzymes that break down barriers.
Simple:
Invasion factors help germs “break in.”
COMMUNITY-ACQUIRED VS NOSOCOMIAL
Community-Acquired Infection
Infection acquired outside of a healthcare setting.
Example:
Getting the flu from school, work, or home.
Nosocomial Infection
Infection acquired in a healthcare setting.
Also called healthcare-associated infection.
Example:
Infection acquired in hospital.
STAGES OF INFECTIOUS DISEASE
1. Incubation Period
Time between exposure and symptom development.
The pathogen is present, but symptoms may not show yet.
Simple:
You were exposed, but you do not feel sick yet.
2. Prodromal Stage
Early symptoms begin.
Symptoms are usually vague.
Examples:
Feeling tired
Mild fever
Headache
Body aches
Simple:
You can tell something is starting, but it is not full-blown yet.
3. Acute Stage
Microbe rapidly multiplies.
Symptoms are strongest.
Disease has maximum impact.
Simple:
This is the “fully sick” stage.
4. Convalescent Stage
Infection is controlled.
Pathogen is being eliminated.
Symptoms improve.
Tissue repair begins.
Simple:
Recovery stage.
5. Resolution
Microbe is fully eliminated.
No remaining signs or symptoms.
Simple:
Infection is gone.
DIAGNOSING INFECTIOUS DISEASE
Diagnosis depends on:
Finding the likely pathogen or evidence of it
Matching symptoms with an infectious process
Criteria for Diagnosis
1. Recovering the Pathogen
Pathogen is found from the infected site.
Examples:
Blood sample
Sputum
Urine
Wound culture
Stool sample
2. Clinical Signs and Symptoms
Symptoms must match an infection.
Examples:
Fever
Redness
Swelling
Pain
Drainage
Cough
Diarrhea
LAB TECHNIQUES FOR DIAGNOSIS
Culture
Growing the microorganism outside the body.
Purpose:
Identify the pathogen.
Help choose treatment.
Serology / Antigen Detection
Looks for immune markers or pathogen antigens.
Simple:
Checks if the body has reacted to the pathogen or if pieces of the pathogen are present.
DNA and RNA Sequencing
Looks for genetic material of the pathogen.
Simple:
Identifies the organism by its genetic code.
HIGH-YIELD STUDY SUMMARY 🚨
Host = organism that supports another organism.
Infectious disease = damage caused by interaction with another organism.
Colonization = microbes present and multiplying, but not always disease.
Virulence = ability to cause disease.
Pathogens = microbes likely to cause disease.
Mutualism = both benefit.
Commensalism = microbe benefits, host unaffected.
Parasitic = microbe benefits, host harmed.
Viruses need host cells to reproduce.
Bacteria reproduce on their own.
Fungi include yeasts and molds.
Protozoa are single-celled parasites.
Helminths are worms.
Prions are misfolded proteins causing brain degeneration.
Incidence = new cases.
Prevalence = active cases.
Endemic = expected in a region.
Epidemic = sudden increase.
Pandemic = spread across continents.
Portal of entry = how pathogen enters.
Toxins damage host cells.
Adhesion factors help microbes stick.
Evasion factors help microbes hide.
Invasion factors help microbes spread.
Incubation = exposed, no symptoms yet.
Prodromal = early symptoms.
Acute = most severe symptoms.
Convalescent = recovery.
Resolution = infection gone.