Principles of Disease and Epidemiology

Definitions and Key Concepts

Understanding disease in microbiology requires precise terminology and concepts. Pathology is the study of disease, while etiology refers to the study of the cause of a disease. Pathogenesis describes the manner in which disease develops. Infection is the invasion or colonization of the body by pathogenic organisms. Epidemiology is the study of the frequency and distribution of disease, and disease is an abnormal state in which the body is not functioning properly.

Historical Epidemiologists

Early epidemiologists made significant contributions to disease control:

• John Snow (1848–1849): Mapped cholera cases in London, pioneering descriptive epidemiology.

• Ignaz Semmelweis (1846–1848): Demonstrated that handwashing reduced puerperal fever.

• Florence Nightingale (1858): Showed improved sanitation decreased epidemic typhus.

Types of Epidemiology

Epidemiology is divided into three main types:

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

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

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

CDC and Disease Surveillance

The Centers for Disease Control and Prevention (CDC) collects and analyzes epidemiological information in the United States. It publishes the Morbidity and Mortality Weekly Report (MMWR) and tracks notifiable infectious diseases, which must be reported by physicians. Key rates include:

• 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.

Normal Microbiota and Human Biome

Normal and Transient Microbiota

The human body hosts a vast array of microorganisms. Normal microbiota permanently colonize the host and are generally harmless, while transient microbiota may be present for days, weeks, or months. Symbiosis describes the relationship between normal microbiota and the host, which can be mutualistic, commensal, or parasitic. Some pathogens are opportunistic, causing disease only under altered conditions.

Distribution of Normal Microbiota

The composition of normal microbiota varies by body region and is influenced by nutrients, physical and chemical factors, host defenses, and mechanical factors. Representative microbiota include:

• Skin: Propionibacterium, Staphylococcus, Corynebacterium, Micrococcus, Candida, Malassezia

• Eyes: Staphylococcus epidermidis, S. aureus, diphtheroids

• Nose and Throat: Staphylococcus aureus, S. epidermidis, Streptococcus pneumoniae, Haemophilus

• Mouth: Streptococcus, Lactobacillus, Actinomyces, Candida

• Large Intestine: Escherichia coli, Bacteroides, Fusobacterium, Lactobacillus

• Urinary and Reproductive Systems: Staphylococcus, Micrococcus, Enterococcus, Lactobacillus, Candida albicans

Symbiotic Relationships

Normal microbiota interact with the host in various ways:

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

• Mutualism: Both organisms benefit.

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

Koch’s Postulates and Disease Classification

Koch’s Postulates

Koch’s postulates are a set of criteria used to establish the 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 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.

Classifying Infectious Diseases

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

• Signs: Changes that can be measured or observed.

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

Occurrence and Spread of Disease

Types of Disease Occurrence

• Endemic: Constantly present in a population (e.g., cold, influenza).

• Epidemic: Unusually large number of cases in a population.

• Pandemic: Epidemic that spreads worldwide (e.g., AIDS).

• Sporadic: Occurs only occasionally.

• Herd immunity: Immunity in most of a population.

Incidence and Prevalence

• Incidence: Number of new cases during a particular time period.

• Prevalence: Total number of cases at a specified time, including old and new cases.

Disease Rate Terms

• Morbidity rate:

• Mortality rate:

• Incidence rate:

• Prevalence rate:

• Seroprevalence: Number of individuals antibody positive for a particular infection.

Severity and Duration of Disease

• Acute disease: Rapid onset, short duration.

• Chronic disease: Slow development, long duration.

• Subacute disease: Intermediate between acute and chronic.

• Latent disease: Inactive for a time, then activates.

Stages of Disease Development

• Incubation period: Interval between initial infection and first signs/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.

Transmission of Disease

Contact Transmission

• Direct contact: Close association between infected and susceptible host (e.g., handshaking, kissing).

• Congenital transmission: From mother to fetus or newborn.

• Indirect contact: Transfer via fomites (inanimate objects).

• Droplet transmission: Airborne droplets less than 1 meter.

Vehicle Transmission

• Airborne: Pathogens carried by dust or droplets suspended in air.

• Waterborne: Pathogens transmitted via contaminated water.

• Foodborne: Pathogens transmitted via contaminated food.

Arthropod Transmission

• Mechanical transmission: Arthropod carries pathogen on feet.

• Biological transmission: Pathogen reproduces in vector.

• Examples: Malaria, plague, Lyme disease (mosquitoes, ticks, fleas).

Healthcare-Associated Infections (HAIs)

Nosocomial Infections

Healthcare-associated infections (HAIs) are acquired while receiving treatment in a healthcare facility. They affect 1 in 25 hospital patients, with over 2 million infections and 70,000 deaths annually. HAIs result from:

• Microorganisms in the hospital environment

• Weakened status of the host

• Chain of transmission in the hospital

Principal Sites of HAIs

• Urinary tract infections

• Surgical site infections

• Lower respiratory infections

• Gastrointestinal infections

• Bloodstream infections

Control of HAIs

• Universal precautions: Standard and transmission-based precautions

• Reduce number of pathogens: Handwashing, disinfecting, cleaning instruments, using disposables

• Infection control committees

Emerging Infectious Diseases

Factors Contributing to Emerging Diseases

• Population growth and urbanization

• International travel

• Transport and migration of animals and food

• Changes in food processing, agriculture

• Human behavior, technology, industry

• Encroachment on wilderness habitats

• Microbial evolution and resistance

• Ecological and climate changes

• Modern medicine (immunosuppression)

• Inadequacy of vaccination, public health

• Social unrest and war

• Bioterrorism

• Virulence enhancing mechanisms

Examples of Emerging Diseases

• Genetic recombination: Escherichia coli O157, avian influenza (H5N1)

• Evolution of new strains: Vibrio cholerae O139

• Antibiotic-resistant strains

• Changes in weather patterns: Hantavirus

• Modern transportation: Zika virus, Chikungunya, West Nile encephalitis

• Insect vectors: Aedes aegypti, A. albopictus

• Ecological disaster, war, expanding human settlement: Coccidioidomycosis

• Animal control measures: Lyme disease

Summary Table: Representative Normal Microbiota by Body Region

Summary Table: Selected Zoonoses

Summary Table: Representative Arthropod Vectors

Summary Table: Microorganisms Involved in Healthcare-Associated Infections

Additional info: These notes expand on brief points with academic context, definitions, and examples to provide a comprehensive study guide for exam preparation in microbiology.