Principles of Disease, Epidemiology, and Infection: Microbiology Study Notes

Pathology, Infection, and Disease

Key Definitions

• Pathology: The study of disease, including its causes, development, and effects on the body.

• Etiology: The study of the cause of a disease.

• Pathogenesis: The process by which disease develops in the host.

• Infection: Colonization of the body by pathogenic microorganisms.

• Disease: An abnormal state in which the body is not functioning normally.

Human Microbiome

Acquisition and Relationships

• The human microbiome is acquired during birth and through environmental exposure.

• Microbiota can be classified as normal (permanent residents) or transient (temporary residents).

• Symbiosis describes the relationship between microbiota and the host, which can be:

  • Commensalism: One organism benefits, the other is unaffected. Example: Bacteria on skin.

  • Mutualism: Both organisms benefit. Example: Escherichia coli in the intestine synthesizes vitamins for the host.

  • Parasitism: One organism benefits at the expense of the other. Example: Pathogenic bacteria causing disease.

• Some normal microbiota are opportunistic pathogens, causing disease under certain conditions.

Normal Microbiota and the Host

Roles and Protection

• Microbial antagonism: Competition between microbes that helps protect the host.

• Normal microbiota protect the host by:

  • Occupying niches that pathogens might otherwise occupy.

  • Producing acids that inhibit pathogen growth.

  • Producing bacteriocins (antimicrobial peptides).

• Probiotics: Live microbes applied to or ingested into the body to exert a beneficial effect.

Body Regions and Microbiota

• Areas containing normal flora: Skin, conjunctiva of the eye, nose & throat (upper respiratory), mouth, large intestine, urinary and reproductive systems (vagina, urethra).

• Areas that are typically sterile: Back of the eye, lower respiratory tract, blood/heart, kidneys, abdominal cavity, thoracic cavity, gonads, uterus during pregnancy.

Table: Representative Normal Microbiota by Body Region

Region	Principal Components	Comments
Skin	Propionibacterium, Staphylococcus, Corynebacterium, Micrococcus, Acinetobacter, Brevibacterium, Candida (fungus), Malassezia (fungus)	Most microbes in direct contact with skin can't become residents; skin secretions and sweat glands have antimicrobial properties.
Eye (Conjunctiva)	Staphylococcus epidermidis, S. aureus, Corynebacterium, Streptococcus, Micrococcus	Conjunctiva contains few microbes due to tears and blinking.
Nose & Throat (Upper Respiratory)	Staphylococcus aureus, S. epidermidis, Streptococcus pneumoniae, Corynebacterium, Haemophilus	Many microbes are transient; mucous and cilia remove many microbes.
Mouth	Streptococcus, Lactobacillus, Actinomyces, Bacteroides, Neisseria, Veillonella, Haemophilus, Corynebacterium, Candida (fungus)	Abundant moisture and nutrients support large and diverse populations.
Large Intestine	Escherichia coli, Bacteroides, Fusobacterium, Lactobacillus, Enterococcus, Clostridium, Proteus, Klebsiella, Candida (fungus)	Largest microbial population; anaerobic environment favors certain bacteria.
Urinary & Reproductive Systems	Staphylococcus, Micrococcus, Enterococcus, Lactobacillus, Streptococcus, Candida (fungus), Trichomonas vaginalis (protozoan)	Urine and vaginal secretions inhibit microbial growth; pH and acidity are important factors.

The Etiology of Infectious Diseases

Koch's Postulates

• Koch's postulates are criteria used to establish a causative relationship between a microbe and a disease.

• The four postulates are:

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

  2. The pathogen must be isolated from the diseased host and grown in pure culture.

  3. The pathogen from the pure culture must cause the disease when inoculated into a healthy, susceptible laboratory animal.

  4. The pathogen must be isolated from the inoculated animal and must be shown to be the original organism.

• Exceptions:

  • Some pathogens cause multiple diseases.

  • Some pathogens only infect humans.

  • Some microbes cannot be cultured in artificial media (e.g., Rickettsia, viruses).

Types of Pathogens

• Non-pathogen: Microorganism that does not cause disease.

• True pathogen: Microorganism that causes disease in healthy individuals.

• Opportunistic pathogen: Microorganism that causes disease only when the host's defenses are compromised.

Classifying Infectious Diseases

Symptoms, Signs, and Syndromes

• Symptom: A change in body function felt by the patient (subjective).

• Sign: A change that can be measured or observed (objective).

• Syndrome: A specific group of signs and symptoms that accompany a disease.

Types of Diseases

• Communicable disease: Can be spread from one host to another.

• Contagious disease: Easily spread from one host to another.

• Noncommunicable disease: Not transmitted from host to host.

Occurrence of Disease

• Incidence: Number of new cases during a specific time period. $\text{Incidence} = \frac{\text{Number of new cases}}{\text{Population at risk}}$

• Prevalence: Total number of cases at a specific time (includes old and new cases). $\text{Prevalence} = \frac{\text{Total number of cases}}{\text{Total population}}$

Frequency of Disease

• Sporadic disease: Occurs occasionally.

• Endemic disease: Constantly present in a population.

• Epidemic disease: Acquired by many people in a short time.

• Pandemic disease: Worldwide epidemic.

Severity or Duration of Disease

• Acute disease: Develops rapidly, lasts a short time (e.g., influenza).

• Chronic disease: Develops slowly, lasts a long time (e.g., tuberculosis).

• Subacute disease: Intermediate between acute and chronic.

• Latent disease: Period of no symptoms; causative agent is inactive, then becomes active (e.g., chickenpox/shingles).

• Herd immunity: Protection of a population from infection due to a high proportion of immune individuals, often achieved through vaccination.

Extent of Host Involvement

Types of Infection

• Local infection: Pathogens are limited to a small area (e.g., boils, abscesses).

• Systemic (generalized) infection: Microorganisms or their products spread throughout the body (e.g., measles).

• Focal infection: Systemic infection that began as a local infection.

• Sepsis: Toxic inflammatory condition from spread of microbes or toxins.

• Septicemia: Multiplication of pathogens in the blood; often fatal.

• Bacteremia: Presence of bacteria in the blood.

• Toxemia: Presence of toxins in the blood.

• Viremia: Presence of viruses in the blood.

• Primary infection: Acute infection causing initial illness.

• Secondary infection: Caused by opportunistic pathogen after primary infection weakens host.

• Subclinical disease: No noticeable signs or symptoms.

Patterns of Disease

Predisposing Factors

• Factors that make the body more susceptible to disease include:

  • Short urethra in females

  • Inherited traits (e.g., sickle cell gene)

  • Climate and weather

  • Fatigue

  • Age

  • Lifestyle

  • Chemotherapy

Stages in the Development of Disease

• Incubation period: Interval between initial infection and first appearance of symptoms.

• Prodromal period: Early, mild symptoms.

• Period of illness: Most severe signs and symptoms.

• Period of decline: Signs and symptoms subside.

• Period of convalescence: Body returns to pre-diseased state.

Disease Process

Syndrome, Signs, and Symptoms

• Every disease is characterized by a syndrome: a set of signs and symptoms.

• Signs: Objective, measurable (e.g., rash, fever).

• Symptoms: Subjective, felt by patient (e.g., pain, headache).

The Spread of Infection

Reservoirs of Infection

• Reservoir: Continual source of disease organism.

• Can be living (humans, animals) or non-living (soil, water).

• Provides a place for pathogen survival and multiplication.

Types of Reservoirs

• Human reservoirs: People who harbor pathogens and transmit them (carriers).

  • Incubation carriers: Transmit during incubation period.

  • Convalescent carriers: Transmit during recovery.

  • Asymptomatic (latent) carriers: No symptoms but can transmit (e.g., HIV, Gonorrhea, Chlamydia).

• Animal reservoirs: Wild and domestic animals (zoonoses).

• Non-living reservoirs: Soil and water.

Table: Selected Zoonoses

Disease	Causative Agent	Animal Reservoir	Transmission to Humans
Rabies	Rabies virus	Bats, foxes, skunks, dogs, coyotes	Bite
Lyme disease	Borrelia burgdorferi	Field mice, deer	Tick bite
Anthrax	Bacillus anthracis	Domestic livestock	Direct contact, ingestion, inhalation
Plague	Yersinia pestis	Rodents	Flea bite
Ringworm	Microsporum, Trichophyton	Domestic animals	Direct contact
Additional info: See textbook for more zoonoses.

Transmission of Disease

Contact Transmission

• Direct contact: Person-to-person transmission (e.g., touching, kissing, sexual contact).

• Indirect contact: Transmission via a nonliving object (fomite) (e.g., tissues, towels, bedding).

• Droplet transmission: Airborne droplets travel short distances (<1 meter) (e.g., sneezing, coughing).

Vehicle Transmission

• Transmission by a medium such as water, food, air, blood, or body fluids.

• Waterborne: Pathogens spread by contaminated water (e.g., cholera).

• Foodborne: Pathogens transmitted by improperly cooked or stored food (e.g., food poisoning).

• Airborne: Pathogens spread by droplets traveling >1 meter (e.g., measles, tuberculosis).

Vector Transmission

• Vectors: Animals (usually arthropods) that carry pathogens from one host to another.

• Mechanical transmission: Passive; arthropod carries pathogen on body parts.

• Biological transmission: Active; pathogen reproduces inside vector and is transmitted via bite.

Table: Representative Arthropod Vectors and the Diseases They Transmit

Disease	Causative Agent	Arthropod Vector
Malaria	Plasmodium spp.	Anopheles mosquito
Lyme disease	Borrelia burgdorferi	Ixodes tick
Plague	Yersinia pestis	Rat flea
Rocky Mountain spotted fever	Rickettsia rickettsii	Dog tick
Chagas disease	Trypanosoma cruzi	Triatoma (kissing bug)
Additional info: See textbook for more vector-borne diseases.

Healthcare-Associated Infections (HAIs)

Definition and Importance

• Healthcare-associated infections (HAIs): Infections acquired while receiving treatment in a healthcare facility.

• Also known as nosocomial infections.

• Affect 1 in 25 hospital patients; 2 million per year infected, over 70,000 deaths.

Causes and Prevention

• Result from:

  • Microorganisms in the hospital environment

  • Weakened status of the host

  • Chain of transmission in the hospital

• Compromised host: Individual whose resistance to infection is impaired by disease, therapy, or burns.

• Prevention methods:

  • Universal precautions

  • Standard precautions (basic, minimum practices)

  • Transmission-based precautions (for known/suspected infections)

  • Handwashing

  • Disinfecting equipment and surfaces

  • Use of disposable items

  • Infection control committees

Table: Principal Sites of Nosocomial Infections

Type of Infection	Comments
Urinary tract infections	Most common; often associated with catheters.
Surgical site infections	Second most common; associated with surgical procedures.
Lower respiratory infections	Often associated with ventilators.
Bacteremia (bloodstream infections)	Associated with intravenous catheters.
Other	Includes skin, gastrointestinal, and other sites.