Principles of Disease and Epidemiology: Microbiology Study Guide

Principles of Disease and Epidemiology

Pathology, Infection, and Disease

Pathology is the scientific study of disease, focusing on its causes, development, and effects on the body. Understanding the terminology is essential for microbiology students:

• Pathology: The study of disease.

• Etiology: The cause of a disease.

• Pathogenesis: The development of disease.

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

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

The Human Microbiome

The human microbiome consists of the collection of microorganisms that inhabit the human body. It begins to establish in utero and is further shaped by exposure to food, people, and pets throughout life.

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

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

• Factors affecting microbiota: Nutrients, physical and chemical factors, host defenses, and mechanical factors.

The Human Microbiome Project analyzes relationships between microbial communities and human health.

Birth and Microbiome

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

• Cesarean birth: Microbiome resembles human skin, with prevalence of Staphylococcus aureus.

Relationships Between Normal Microbiota and the Host

Normal microbiota interact with the host in various ways, often providing protection against pathogens.

• Microbial antagonism (competitive exclusion): Competition between microbes for nutrients and space.

• Protection mechanisms: Competing for nutrients, producing substances harmful to invading microbes, affecting pH and available oxygen.

Types of Symbiosis

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

• Mutualism: Both organisms benefit.

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

• Opportunistic pathogens: Normal microbiota that can cause disease under certain conditions.

Koch’s Postulates

Establishing Disease Causation

Koch’s postulates are a set of 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 include pathogens that cause multiple diseases, those only affecting humans, and microbes that cannot be 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 host to host.

Occurrence of Disease

Incidence and Prevalence

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

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

Patterns of Disease Occurrence

• 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: 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.

• Herd immunity: Immunity in most of a population.

Extent of Host Involvement

• Local infection: Limited to a small area.

• Systemic infection: Throughout the body.

• Focal infection: Systemic infection that began locally.

• Sepsis: Toxic inflammatory condition from spread of microbes.

• Bacteremia: Bacteria in the blood.

• Septicemia: Growth of bacteria in the blood.

• Toxemia: Toxins in the blood.

• Viremia: Viruses in the blood.

• Primary infection: Initial acute infection.

• Secondary infection: Opportunistic infection after primary infection.

• Subclinical disease: No noticeable signs or symptoms.

Predisposing Factors

Factors that make the body more susceptible to disease include:

• Gender

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

• Climate and weather

• Lack of vaccination

• Fatigue

• Age

• Lifestyle

• Nutrition

• Chemotherapy

Development of Disease

The progression of disease follows distinct stages:

• Incubation period: Interval between 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.

Reservoirs of Infection

Reservoirs are continual sources of infection:

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

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

• Nonliving reservoirs: Soil and water.

Transmission of Disease

Contact Transmission

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

• Congenital transmission: From mother to fetus or newborn.

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

• Droplet transmission: Airborne droplets less than 1 meter.

Vehicle Transmission

• Airborne: Pathogens carried by air.

• Waterborne: Pathogens transmitted through contaminated water.

• Foodborne: Pathogens transmitted through contaminated food.

Vector Transmission

• Mechanical transmission: Arthropod carries pathogen on its feet.

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

Healthcare-Associated Infections (HAIs)

Overview and Causes

HAIs are infections acquired while receiving treatment in a healthcare facility. They affect 1 in 25 hospital patients, with over 2 million cases and 70,000 deaths annually in the U.S.

• Causes: Microorganisms in hospital environment, weakened host, chain of transmission.

• Compromised host: Resistance to infection is impaired by disease, therapy, or burns.

Principal Sites of HAIs

Microorganisms Involved in HAIs

Microorganism	Most Common Infection Type	Percentage of Total Infections	Percentage Resistant to Antibiotics
Coagulase-negative staphylococci	Bloodstream	11%	Not reported
Staphylococcus aureus	Surgical site	10%	55%
Clostridium difficile	Diarrhea after abdominal surgery	12%	Not reported
Enterococcus spp.	Urinary tract infections	12%	30%
Candida spp. (fungi)	Urinary tract infections	10%	20%
Pseudomonas aeruginosa	All sites	9%	20%
Klebsiella pneumoniae	All sites	6%	20%
Escherichia coli	All sites	5%	20%
Acinetobacter baumannii	All sites	2%	68%

Control of HAIs

• Universal precautions: Basic minimum practices.

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

• Reducing pathogens: Handwashing, disinfecting, cleaning instruments, using disposable items, infection control committees.

Emerging Infectious Diseases

Definition and Contributing Factors

Emerging infectious diseases are new, increasing in incidence, or have potential to increase. Most are zoonotic, viral, and vector-borne.

• Genetic recombination: e.g., Escherichia coli O157, avian influenza H5N1.

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

• Antibiotic resistance: Due to widespread use of antibiotics and pesticides.

• Weather changes: e.g., Hantavirus.

• Modern transportation: e.g., Zika virus, West Nile encephalitis.

• Ecological disaster, war, human settlement: e.g., Coccidioidomycosis.

• Animal control measures: e.g., Lyme disease.

• Public health failure: e.g., Diphtheria.

• Bioterrorism: Deliberate spread of disease.

Epidemiology

Definition and Role

Epidemiology is the study of where and when diseases occur and how they are transmitted in populations. Epidemiologists determine etiology, identify factors, develop control methods, and assemble data.

Epidemiological Graphs

Types of Epidemiology

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

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

• Experimental epidemiology: Hypothesis and controlled experiments (Ignaz Semmelweis).

• Clinical trial: Test and control group.

Scientist	Years	Contribution
John Snow	1848–1849	Mapped the occurrence of cholera in London
Ignaz Semmelweis	1846–1848	Showed that handwashing decreased the incidence of puerperal sepsis
Florence Nightingale	1858	Showed that improved sanitation decreased the incidence of epidemic typhus

The Centers for Disease Control and Prevention (CDC)

Role and Reporting

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

• Publishes Morbidity and Mortality Weekly Report (MMWR).

• Morbidity: Incidence of a specific notifiable disease.

• Mortality: 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.

Nationally Notifiable Infectious Diseases

Disease	Notes
Anthrax	Severe acute respiratory syndrome-associated coronavirus disease
Botulism	Hepatitis A, B, and C
Brucellosis	HIV infection
Cholera	Influenza-associated pediatric mortality
Cryptosporidiosis	Legionellosis
Dengue	Lyme disease
Giardiasis	Malaria
Haemophilus influenzae	Measles
Hantavirus	Mumps
Hemolytic uremic syndrome	Pertussis
Hepatitis	Plague
Influenza	Polio
Legionellosis	Rabies
Leptospirosis	Salmonellosis
Listeriosis	Shigellosis
Malaria	Streptococcus
Measles	Tetanus
Mumps	Toxic shock syndrome
Pertussis	Trichinellosis
Plague	Tuberculosis
Polio	Typhoid fever
Rabies	Varicella
Salmonellosis	Yellow fever

Clinical Focus: Healthcare-Associated Infections

Blood cultures grown on mannitol-salt agar; coagulase-positive; gram-positive cocci. Methicillin-resistant Staphylococcus aureus (MRSA) is a major concern:

• USA100: 92% of healthcare strains.

• USA300: 89% of community-acquired strains.

Example: MRSA infections are difficult to treat due to antibiotic resistance and are a significant cause of morbidity and mortality in healthcare settings.