Principles of Disease and Epidemiology: Pathology, Infection, and Disease

Pathology, Infection, and Disease

Key Concepts in Disease

Understanding the principles of disease is fundamental in microbiology. The following terms are essential for describing the process and impact of infectious diseases:

• 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 mechanism by which disease develops.

• 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

Normal and Transient Microbiota

The human body is colonized by a diverse community of microorganisms, known as the normal microbiota. These microbes play a crucial role in health and disease.

• Transient microbiota: Present for short periods (days, weeks, or months).

• Normal microbiota: Permanently colonize the host without causing disease under normal conditions.

• Symbiosis: The relationship between normal microbiota and the host, which can be mutualistic, commensal, or parasitic.

• Microbial antagonism (competitive exclusion): Normal microbiota compete with potential pathogens, protecting the host.

Types of Symbiotic Relationships

• 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 (e.g., Escherichia coli, Neisseria meningitidis, Streptococcus pneumoniae).

Protective Roles of Normal Microbiota

• Occupy niches that pathogens might occupy.

• Produce acids that inhibit pathogen growth.

• Produce bacteriocins (antimicrobial peptides).

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

The Etiology of Infectious Diseases

Koch’s Postulates

Koch’s postulates are a set of criteria used to establish a causative relationship between a microbe and a disease:

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 cultured pathogen must cause the disease when introduced into a healthy, susceptible host.

4. The pathogen must be re-isolated from the experimentally infected host and shown to be the original organism.

Applications: Used to demonstrate that Bacillus anthracis causes anthrax and Mycobacterium tuberculosis causes tuberculosis.

Limitations: Some pathogens cause multiple diseases, only infect humans, or cannot be cultured in the lab.

Classifying Infectious Diseases

Definitions

• Symptom: A subjective change in body function felt by the patient (e.g., pain, fatigue).

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

• 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 and Prevalence

• Incidence: Number of new cases in a population during a specific time period (indicates spread).

• Prevalence: Total number of cases (new and existing) at a given time (indicates how widespread and long-lasting a disease is).

Patterns of Disease Occurrence

• Sporadic disease: Occurs occasionally.

• Endemic disease: Constantly present in a population.

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

• Pandemic disease: Worldwide epidemic.

• Herd immunity: Immunity in most of a population reduces disease spread.

Severity and Duration of Disease

• Acute disease: Symptoms develop rapidly.

• Chronic disease: Develops slowly, persists for a long time.

• Subacute disease: Intermediate between acute and chronic.

• Latent disease: Causative agent is inactive for a time but can reactivate.

Extent of Host Involvement

• Local infection: Pathogens are limited to a small area.

• Systemic infection: Infection spreads throughout the body.

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

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

• Bacteremia: Bacteria in the blood.

• Septicemia: Growth of bacteria in the blood (blood poisoning).

• Toxemia: Toxins in the blood.

• Viremia: Viruses in the blood.

• Primary infection: Acute infection causing the initial illness.

• Secondary infection: Opportunistic infection after a primary infection.

• 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 of Disease

• Incubation period: Interval between infection and first symptoms.

• Prodromal period: Mild signs or symptoms appear.

• Period of illness: Most severe signs and symptoms.

• Period of decline: Signs and symptoms subside.

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

The Spread of Infection

Reservoirs of Infection

• Human reservoirs: Carriers may have inapparent infections or latent diseases (e.g., AIDS, gonorrhea).

• Animal reservoirs: Zoonoses are diseases transmitted from animals to humans (e.g., rabies, Lyme disease).

• Nonliving reservoirs: Soil and water (e.g., botulism, tetanus).

Transmission of Disease

• Contact transmission:

  • Direct contact: Physical contact between infected and susceptible host.

  • Indirect contact: Spread by fomites (inanimate objects).

  • Droplet transmission: Transmission via airborne droplets.

• Vehicle transmission: Transmission by an inanimate reservoir (food, water, air).

• Vector transmission: Arthropods (e.g., fleas, ticks, mosquitoes) transmit disease by:

  • Mechanical transmission: Pathogen carried on vector's body parts.

  • Biological transmission: Pathogen reproduces in vector and is transmitted via bite.

Nosocomial (Healthcare-Associated) Infections

Overview

Nosocomial infections are acquired during a hospital stay and affect 5–15% of all hospital patients. They result from a combination of microorganisms in the hospital environment, a compromised host, and the chain of transmission.

Common Causes and Sites of Nosocomial Infections

Type of Infection	Comment
Urinary tract infections	Most common; often related to urinary catheterization.
Surgical site infections	Second most common; associated with post-operative wounds.
Lower respiratory infections	Includes pneumonia, often associated with ventilators.
Bacteremia	Often caused by intravenous catheterization.
Other	Includes gastrointestinal and skin infections.

Emerging Infectious Diseases

Definition and Contributing Factors

Emerging infectious diseases are those that are new, increasing in incidence, or have the potential to increase in the near future. Contributing factors include:

• Genetic recombination (e.g., E. coli O157, avian influenza H5N1)

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

• Inappropriate use of antibiotics and pesticides (leading to resistance)

• Modern transportation (rapid spread of pathogens)

• Ecological changes, war, and expanding human settlement

• Animal control measures

• Public health failures

Epidemiology

Definition and Scope

Epidemiology is the study of the distribution and determinants of health-related states and events in populations, and the application of this study to control diseases. The Centers for Disease Control and Prevention (CDC) is a key organization in the United States for epidemiological surveillance and reporting.

Types of Epidemiological Studies

• Descriptive: Collection and analysis of data regarding the occurrence of disease.

• Analytical: Comparison of a diseased group and a healthy group to identify causes and risk factors.

• Experimental: Controlled experiments to test hypotheses about disease causation.

Case Reporting and Notifiable Diseases

• Healthcare workers report specified diseases to local, state, and national offices.

• Nationally notifiable diseases must be reported by physicians.

Morbidity and Mortality

• Morbidity: Incidence of a specific notifiable disease.

• Mortality: Number of deaths from notifiable diseases.

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

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

Example: The CDC publishes the Morbidity and Mortality Weekly Report (MMWR) to track and analyze disease trends in the United States.