Infection, Disease & Epidemiology
Lesson focus - Explain host-microbe relationships, identify stages and patterns of infection, compare transmission routes, and apply infection prevention principles in healthcare settings.
Host-Microbe Relationship
Symbiosis
Symbiosis is a close relationship between two different types of organisms in a community.
| Type | Relationship |
|---|---|
| Mutualism | Both members benefit from the interaction |
| Commensalism | One organism benefits; the other is neither harmed nor helped |
| Parasitism | One organism benefits; the other is harmed |
A pathogen is a parasite capable of causing disease.
Normal Flora
Normal flora are microorganisms that are always present on or in a person and usually do not cause disease.
Normal flora may be found on or in:
- Skin
- Mucous membranes
- Eyes
- Mouth
- Digestive tract
- Respiratory tract
Normal flora help protect the host by:
- Impeding colonization by other bacteria
- Depleting nutrients and making survival difficult for others through microbial competition
- Making the environment too acidic for colonization
Example: lysosomes in tears help protect against colonization.
Opportunistic Pathogens
Opportunistic pathogens are microorganisms that become pathogenic when they take advantage of opportunities that are not normally available in the host.
Examples and risk situations include:
- Pseudomonas aeruginosa in cystic fibrosis patients
- Acute and chronic diseases
- Malnutrition
- Emotional and physical stress
- Older and younger age groups
- Radiation or chemotherapy
- Immunosuppressive drugs used in transplant patients
Changes in Normal Flora
Opportunistic pathogens can cause infection when normal flora changes.
- Candida albicans may cause yeast infection after prolonged antibiotic use.
- Hormonal changes, stress, diet changes, or exposure to excess organisms can allow other bacteria to thrive.
- A normal flora organism can cause infection when it reaches a body site it does not normally inhabit.
Example: E. coli from the GI tract entering the urethra can cause a UTI.
Injury, burns, and surgery can create opportunities for infection.
Stages of Infection
Portal of Entry
The portal of entry is the site where the pathogen enters the body.
Common portals include:
- Skin
- Mucous membranes
- Placenta
Parenteral entry means pathogens are introduced directly into subcutaneous tissue.
Virulence, Pathogenicity, and Virulence Factors
- Virulence - degree of disease-evoking power of a microbe based on virulence factors.
- Pathogenicity - ability to cause disease.
- Virulence factors - factors that enhance the potential to cause disease.
Invasion
Invasion is the ability to break down primary and secondary defensive barriers of the host.
Adhesion
- First step in infection that allows colonization.
- Fimbriae, pili, and receptor sites enable adhesion.
Colonization
- Pathogen begins to replicate once a foothold is established.
- Common colonization sites:
- Urogenital tract
- Digestive tract
- Respiratory tract
- Conjunctiva of the eye
Evasion
Evasion is the ability to avoid the host response.
Examples include:
- Slime layer or capsule preventing phagocytosis
- Production of proteins that bind to host cell antibodies
- Mutation of the organism
Exotoxins
- Proteins secreted by bacterial cells
- Specific to the microbe
- Often specific to the substrate where they bind
- Often named based on where they cause damage
Example: Clostridium tetani produces tetanus. Neurotoxins damage nervous tissue.
Endotoxins
- Released from cell walls during lysis
- Release may be initiated by antibiotic exposure or host defense
- Less specific to the site of infection
- Can act at remote sites
Infectious Dose
Infectious dose is the amount of cells needed to infect the host and cause disease.
Portals of Exit
Portals of exit are where the pathogen leaves the infected patient.
Examples include:
- Excretion or secretion
- Sneeze or cough
- Blood
Etiology: Patterns of Infection
| Pattern | Description |
|---|---|
| Local infection | Organism enters the body and stays confined to a specific tissue, such as warts or fungal skin infections |
| Focal infection | Pathogen spreads from a local infection to other tissue |
| Systemic infection | Infection spreads to several sites and tissue fluids, usually through the circulatory system |
| Septicemia / sepsis | Infection of the blood |
Other Types of Infection
| Type | Description |
|---|---|
| Mixed infection | Several infectious agents establish infection at the same time |
| Chronic infection | Less severe symptoms that persist for long periods |
| Acute infection | Appears rapidly, causes severe symptoms, and vanishes rapidly |
| Primary infection | Initial infection that may be followed by complications from another microbe |
| Secondary infection | Follows a primary infection and may be caused by opportunistic bacteria |
Reservoirs
A reservoir is the source of infection.
Animal Reservoirs
- Transmitted through contact with the animal, animal waste, or waste-contaminated food or water.
- Vectors are animals or insects that carry disease-causing pathogens.
Human Reservoirs
- The human body carries the pathogen.
- The patient may show signs of infection or may be asymptomatic.
- Carriers are infected but asymptomatic and can infect others without knowing.
Nonliving Reservoirs
- Soil
- Water
- Food
Contact Transmission
Direct Contact
Direct contact transmission occurs through direct physical contact with the infectious agent. No intermediate object is involved.
Examples:
- Kissing
- Touching
- Intercourse
Indirect Contact
Indirect contact transmission occurs when a pathogen moves from reservoir to host by a fomite.
A fomite is an inanimate object, such as:
- Stethoscope
- Towel
- Shared item
Droplet Transmission
Droplet transmission occurs when infectious agents travel through respiratory droplets less than 1 meter, or about 3 feet.
Examples:
- Coughing
- Laughing
- Sneezing
- Talking
Associated diseases include:
- Common cold
- Flu
- Pertussis
Droplet precautions:
- Place patient in a private room if possible.
- Cohort if necessary.
- Perform hand hygiene.
- Don a surgical mask before entering the room.
Vehicle Transmission
Airborne Transmission
Airborne transmission spreads pathogens through droplet nuclei, aerosols, and dust that can travel more than 1 meter.
Airborne precautions:
- Negative pressure room
- HEPA filtration with 6 to 12 air changes per hour
- Keep room door closed
- Patient remains in room
- Cohort if a private room is unavailable
Waterborne Transmission
- Associated with untreated or poorly treated sewage.
Foodborne Transmission
- Pathogens are in or on foods that are incompletely cooked, poorly processed, or not properly refrigerated.
Bodily Fluid Transmission
Can involve:
- Blood
- Saliva
- Urine
- Other bodily fluids
Associated diseases include:
- AIDS
- Hepatitis
- Herpes
Vector Transmission
Vector transmission comes from animals.
| Vector type | Description |
|---|---|
| Biological vector | Transmits pathogen through bites, such as mosquitoes or lice |
| Mechanical vector | Carries pathogens passively by feet or other body parts but does not necessarily host the pathogen |
Healthcare-Associated Infections
Healthcare-associated infections, or HAIs, are also called nosocomial infections.
They are infections acquired during treatment in a hospital-like setting but secondary to the patient's original condition.
| Type | Source |
|---|---|
| Exogenous | Shed from other sick individuals |
| Endogenous | Caused by the patient's normal flora |
| Iatrogenic | Results from medical procedures such as catheters or surgery |
Antimicrobial Resistance
Due to mutations, some pathogens are resistant to treatments that once worked.
Control and Prevention
Core Practices
- Disinfection
- Proper hand hygiene
- Surgical asepsis
- Isolation
- Cohorting, or grouping patients with similar conditions
- Standard precautions for all patients in all healthcare settings all of the time
- Proper transmission-based precautions
Hand Hygiene Timing
Perform hand hygiene:
- Before and after patient contact
- Immediately after removing gloves
- When indicated to avoid cross-contamination
- After touching body fluids or contaminated items
- Between tasks and procedures on the same patient when contamination of different body sites is possible
Use approved alcohol-based products unless hands are visibly soiled. If hands are visibly soiled, use soap and water.
Review
- Host-microbe relationship
- Stages of infection
- Epidemiology
- Healthcare-associated infections
- Control and prevention