Principles of Disease and Epidemiology

Introduction

This chapter explores the fundamental concepts of disease, epidemiology, and the human microbiome. It covers definitions, classifications, and mechanisms of disease, as well as the acquisition and role of the human microbiome in health and disease.

Pathology, Infection, and Disease

Key Definitions

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

• Etiology: The cause of a disease.

• Pathogenesis: The development and progression of disease.

• Infection: Invasion or colonization of the body by pathogens.

• Disease: An abnormal state in which the body is not performing normal functions.

Objectives of Pathology

• Identify the cause of disease (etiology).

• Understand the mechanisms of disease development (pathogenesis).

• Describe the structural and functional changes caused by disease.

The Human Microbiome

Acquisition and Composition

The human microbiome is established in utero and further acquired from food, people, and pets. It remains throughout life and is influenced by various factors.

• Normal microbiota: Permanently colonize the host and do not cause disease under normal conditions.

• Transient microbiota: May be present for days, weeks, or months.

• Human Microbiome Project: Analyzes relationships between microbial communities and human health.

Factors Determining Microbiota Distribution

• Nutrients

• Physical and chemical factors

• Host defenses

• Mechanical factors

Microbiome and Birth

• Vaginal birth: Microbiome is dominated by Lactobacillus and Bacteroides.

• Caesarean birth: Microbiome resembles human skin, with Staphylococcus aureus prevalent.

Microbial Antagonism and Host Protection

• Normal microbiota protect the host by competing for nutrients, producing substances harmful to invading microbes, and affecting pH and oxygen availability.

Symbiosis and Types of Microbial 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 can be opportunistic pathogens.

Etiology of Infectious Diseases

Koch’s Postulates

Koch’s postulates are criteria used to prove the cause of an infectious 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 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 shown to be the original organism.

Exceptions to Koch’s Postulates

• Some pathogens cause several disease conditions.

• Some pathogens cause disease only in humans.

• Some microbes have never been cultured.

Classifying Infectious Diseases

Symptoms, Signs, and Syndromes

• Symptoms: Changes in body function felt by the patient.

• Signs: Changes that can be measured or observed.

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

Types of Diseases

• Communicable disease: Spread from one host to another.

• Contagious disease: Easily and rapidly spread.

• Noncommunicable disease: Not spread from one host to another.

Occurrence of Disease

• Incidence: Number of people who develop a disease during a particular time period.

• Prevalence: Number of people who have a disease at a specified time, including old and new cases.

Frequency of Disease

• Sporadic disease: Occurs only occasionally.

• Endemic disease: Constantly present in a population.

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

• Pandemic disease: Worldwide epidemic.

Severity or Duration of Disease

• Acute disease: Symptoms develop rapidly, short duration.

• Chronic disease: Symptoms develop slowly, long duration.

• Subacute disease: Intermediate between acute and chronic.

• Latent disease: Causative agent is inactive for a time, then activates.

• Herd immunity: Immunity in most of a population.

Extent of Host Involvement

Types of Infection

• Local infection: Pathogens limited to a small area.

• Systemic infection: Infection throughout the body.

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

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

• Bacteremia: Bacteria in the blood.

• Septicemia: Growth of bacteria in the blood.

• Toxemia: Toxins in the blood.

• Viremia: Viruses in the blood.

• Primary infection: Acute infection causing initial illness.

• Secondary infection: Opportunistic infection after a primary infection.

• Subclinical disease: No noticeable signs or symptoms.

Patterns of Disease

Predisposing Factors

• Gender

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

• Climate and weather

• Lack of vaccination

• Fatigue

• Age

• Lifestyle

• Nutrition

• Chemotherapy

Stages of Disease

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

• Prodromal period: Early, mild symptoms.

• Period of illness: Disease is most severe.

• Period of decline: Signs and symptoms subside.

• Period of convalescence: Body returns to prediseased state.

The Spread of Infection

Reservoirs of Infection

• Human reservoirs: Carriers may have inapparent or latent diseases.

• Animal reservoirs: Zoonoses are diseases transmitted from animals to humans.

• Nonliving reservoirs: Soil and water.

Transmission of Disease

• Direct contact transmission: Close association between infected and susceptible host.

• Congenital transmission: From mother to fetus or newborn.

• Indirect contact transmission: Via nonliving object (fomite).

• Droplet transmission: Airborne droplets less than 1 meter.

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

• Vector transmission: Arthropods transmit disease by mechanical (pathogen on feet) or biological (pathogen reproduces in vector) methods.

Healthcare-Associated Infections (HAIs)

Definition and Importance

• Acquired while receiving treatment in a healthcare facility.

• Also known as nosocomial infections.

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

Factors Contributing to HAIs

• Microorganisms in the hospital environment.

• Weakened status of the host.

• Chain of transmission in the hospital.

Prevention of HAIs

• Universal and standard precautions.

• Transmission-based precautions (contact, droplet, airborne).

• Handwashing, disinfecting, cleaning instruments, using disposable items.

• Infection control committees.

Emerging Infectious Diseases

Definition and Contributing Factors

• Diseases that are new, increasing in incidence, or have potential to increase.

• Most are zoonotic, viral, and vector-borne.

• Contributing factors include genetic recombination, evolution of new strains, antibiotic resistance, changes in weather, modern transportation, insect vectors, ecological disaster, animal control, public health failure, and bioterrorism.

Epidemiology

Definition and Types of Investigations

• Epidemiology: Study of where and when diseases occur and how they are transmitted in populations.

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

• Analytical epidemiology: Analyzes disease to determine probable cause (e.g., Florence Nightingale).

• Experimental epidemiology: Involves hypothesis and controlled experiments (e.g., Ignaz Semmelweis).

CDC and Disease Reporting

• The Centers for Disease Control and Prevention (CDC) collects and analyzes epidemiological information in the United States.

• Publishes Morbidity and Mortality Weekly Report (MMWR).

• Morbidity: Incidence of a specific notifiable disease.

• Mortality: Deaths from notifiable diseases.

• Notifiable infectious diseases: Diseases physicians are required to report.

• 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.

Clinical Focus: Healthcare-Associated Infections

Example: MRSA

• Methicillin-resistant Staphylococcus aureus (MRSA): Major cause of healthcare-associated infections.

• Strain USA100: 92% of healthcare strains.

• Strain USA300: 89% of community-acquired strains.

Summary Table: Representative Normal Microbiota by Body Region