Principles of Disease, Epidemiology, and Host-Microbe Interactions

Pathology, Infection, and Disease

Key Definitions

• Pathology: The scientific 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: The colonization of the body by pathogenic microorganisms.

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

Normal Microbiota and the Host

Types of Microbiota

• Normal microbiota: Microorganisms that permanently colonize the host without causing disease under normal conditions.

• Transient microbiota: Microbes that are present in the body for a short period (days, weeks, or months) and then disappear.

Symbiosis

• Symbiosis: The relationship between normal microbiota and the host.

• Commensalism: One organism benefits, the other is unaffected.

• Mutualism: Both organisms benefit.

• Parasitism: One organism benefits at the expense of the other.

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

Microbial Antagonism and Host Protection

• Microbial antagonism: Competition between microbes for resources and space.

• Normal microbiota protect the host by:

  • Occupying niches that pathogens might occupy

  • Producing acids

  • Producing bacteriocins (antimicrobial peptides)

• Probiotics: Live microbes administered to confer a health benefit to the host.

Distribution of Normal Microbiota

• Normal flora are found in the skin, conjunctiva, nose & throat, mouth, large intestine, and urinary/reproductive systems.

• Some body sites are sterile (free of microbes): back of the eye, lower respiratory tract, blood/heart, kidneys, abdominal and thoracic cavities, gonads, and uterus during pregnancy.

Koch’s Postulates

Steps to Prove Causation of Disease

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

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

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

• The pathogen must be re-isolated from the inoculated animal and shown to be the original organism.

• Exceptions: Some pathogens cause multiple diseases, only infect humans, or cannot be cultured (e.g., Rickettsia, viruses).

Types of Pathogens

• Non-pathogen: Does not cause disease in healthy individuals.

• True pathogen: Causes disease in healthy hosts.

• Opportunistic pathogen: Causes disease only when the host's defenses are compromised.

Classifying Infectious Diseases

Symptoms, Signs, and Syndromes

• Symptom: Subjective change in body function felt by the patient (e.g., pain, nausea).

• Sign: Objective change that can be measured or observed (e.g., rash, fever).

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

Communicability

• Communicable disease: 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.

• Prevalence: Total number of cases (old and new) at a specific time.

• Sporadic disease: Occurs occasionally.

• Endemic disease: Constantly present in a population.

• Epidemic disease: Many people acquire the disease in a short time.

• Pandemic disease: Worldwide epidemic.

Severity or Duration of Disease

• Acute disease: Rapid onset, short duration (e.g., influenza).

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

• Subacute disease: Intermediate between acute and chronic.

• Latent disease: Periods of inactivity; agent becomes active later (e.g., chickenpox/shingles).

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

Extent of Host Involvement

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

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

• Focal infection: Local infection spreads to another part of the body.

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

• Septicemia: Systemic infection from multiplication of pathogens in blood; often fatal.

• Bacteremia: Presence of bacteria in blood.

• Toxemia: Presence of toxins in blood (e.g., tetanus).

• Viremia: Presence of viruses in blood.

• Primary infection: Acute infection causing initial illness.

• Secondary infection: Caused by opportunistic pathogen after primary infection weakens host (e.g., HIV patients).

• Subclinical disease: No noticeable signs or symptoms (e.g., poliovirus, hepatitis A).

Predisposing Factors

• Factors that increase susceptibility to disease include:

  • Short urethra in females

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

  • Climate and weather

  • Fatigue

  • Age

  • Lifestyle

  • Chemotherapy

Development of Disease

• The sequence of disease development:

  1. Incubation period

  2. Prodromal period

  3. Period of illness

  4. Period of decline

  5. Period of convalescence

• Each stage has characteristic signs and symptoms.

Reservoirs of Infection

• Reservoir: Continual source of disease organisms, allowing survival and multiplication.

• Types:

  • Human reservoirs: People with active disease or carriers (incubation, convalescent, asymptomatic).

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

  • Non-living reservoirs: Soil and water.

Transmission of Disease

Contact Transmission

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

• Indirect contact: Transmission via nonliving object (fomite).

• Droplet transmission: Short-range airborne droplets (less than 1 meter).

Vehicle Transmission

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

• Waterborne: Contaminated water (e.g., cholera).

• Foodborne: Improperly prepared or stored food (e.g., food poisoning).

• Airborne: Droplets or dust traveling more than 1 meter (e.g., tuberculosis).

Vector Transmission

• Vectors: Animals (often arthropods) that transmit pathogens.

• Mechanical transmission: Passive transport on body parts.

• Biological transmission: Pathogen reproduces in vector; transmitted via bite or feces.

Healthcare-Associated Infections (HAIs)

• Infections acquired during treatment in a healthcare facility (nosocomial infections).

• Affect 1 in 25 hospital patients; significant cause of morbidity and mortality.

• Result from microorganisms in the hospital, compromised hosts, and chain of transmission.

• Compromised host: Individual with impaired resistance to infection.

Prevention of HAIs

• Universal and standard precautions (handwashing, disinfection, disposable equipment).

• Transmission-based precautions for known/suspected infections (contact, droplet, airborne).

• Infection control committees monitor and enforce protocols.

Emerging Infectious Diseases (EIDs)

• New or changing diseases increasing in incidence.

• About 75% are zoonotic, often viral/vector-borne.

• Contributing factors:

  • Genetic recombination (e.g., avian influenza H5N1)

  • Evolution of new strains (e.g., Vibrio cholerae O139)

  • Widespread use of antibiotics/pesticides (antibiotic resistance)

  • Climate change, modern transportation, ecological changes, public health failures, bioterrorism

Epidemiology

• The study of where, when, and how diseases occur and are transmitted in populations.

• Epidemiologists determine etiology, spread, and control methods; assemble data and graphs.

Types of Epidemiologic Investigations

• Descriptive epidemiology: Collection and analysis of data (e.g., John Snow).

• Analytical epidemiology: Determines probable cause (e.g., Florence Nightingale).

• Experimental epidemiology: Hypothesis and controlled experiments (e.g., Semmelweis).

Centers for Disease Control and Prevention (CDC)

• Collects and analyzes epidemiological data in the U.S.

• Publishes Morbidity and Mortality Weekly Report (MMWR).

• Morbidity: Incidence of a specific notifiable disease.

• Mortality: Deaths from notifiable diseases.

• Notifiable infectious diseases: Diseases that must be reported by physicians.

• Morbidity rate: Number of people affected in relation to the population in a given time.

• Mortality rate: Number of deaths from a disease in relation to the population in a given time.