Principles of Disease and Epidemiology: Microbiology Study Guide

Principles of Disease and Epidemiology

Pathology, Infection, and Disease

The study of disease in microbiology involves understanding the causes, development, and effects of diseases on the host organism. Key terms are essential for distinguishing between different aspects of disease.

• Pathology: The scientific study of disease, including its causes and effects.

• Etiology: The cause or origin of a disease.

• Pathogenesis: The process by which disease develops.

• Infection: The invasion or colonization of the body by pathogenic microorganisms.

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

The Human Microbiome

The human microbiome consists of the collection of microorganisms that inhabit various regions of the human body. These microbes play crucial roles in health and disease.

• Acquisition: Microbiome begins to establish in utero and is further acquired from food, people, and pets.

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

• Transient microbiota: Present for days, weeks, or months, but do not permanently colonize.

• Distribution and composition: Determined by nutrients, physical and chemical factors, host defenses, and mechanical factors.

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

Representative Normal Microbiota by Body Region

Microbiota composition varies by body region, influenced by environmental and physiological factors.

Region	Principal Components	Comments
Skin	Propionibacterium, Staphylococcus, Corynebacterium, Micrococcus, Acinetobacter, Brevibacterium; Candida (fungus), Malassezia (fungus)	Antimicrobial properties of sweat/oil, low pH, low moisture
Eyes (Conjunctiva)	Staphylococcus epidermidis, S. aureus, diphtheroids, Propionibacterium, Corynebacterium, streptococci, Micrococcus	Tears and blinking remove/inhibit microbes
Nose and Throat	Staphylococcus aureus, S. epidermidis, aerobic diphtheroids, Streptococcus pneumoniae, Haemophilus, Neisseria	Microbial antagonism, nasal secretions, mucus, ciliary action
Mouth	Streptococcus, Lactobacillus, Actinomyces, Bacteroides, Veillonella, Neisseria, Haemophilus, Fusobacterium, Treponema, Staphylococcus, Corynebacterium, Candida (fungus)	Moisture, warmth, food, salivary flow, antimicrobial substances
Large Intestine	Escherichia coli, Bacteroides, Fusobacterium, Lactobacillus, Enterococcus, Bifidobacterium, Enterobacter, Citrobacter, Proteus, Klebsiella, Candida (fungus)	Largest numbers due to moisture/nutrients, mucus, antimicrobial chemicals
Urinary and Reproductive Systems	Staphylococcus, Micrococcus, Enterococcus, Lactobacillus, Bacteroides, aerobic diphtheroids, Pseudomonas, Klebsiella, Proteus, Candida albicans (fungus), Trichomonas vaginalis (protozoan)	Acid-tolerant population, urine flow, pH, cilia, mucus, acidity

Microbiome and Birth

The method of birth influences the initial microbiome composition in newborns.

• Vaginal birth: Microbiome dominated by Lactobacillus and Bacteroides.

• Cesarean birth: Microbiome resembles human skin, dominated by Staphylococcus aureus.

Relationships Between Normal Microbiota and the Host

Normal microbiota interact with the host in various ways, providing protection and sometimes causing disease.

• Microbial antagonism: Competition between microbes, protecting the host by competing for nutrients, producing harmful substances, and affecting pH and oxygen.

• Symbiosis: Relationship between microbiota and host, classified as:

  • 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.

The Etiology of Infectious Diseases: Koch’s Postulates

Koch’s postulates are a set of criteria used to establish the causative relationship between a microbe and a 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 laboratory animal.

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

Exceptions: Some pathogens cause multiple diseases, some only infect humans, and some cannot be cultured.

Classifying Infectious Diseases

Diseases are classified based on their communicability, frequency, and severity.

• Symptoms: Subjective changes felt by the patient.

• Signs: Objective changes measurable or observable.

• Syndrome: Specific group of signs and symptoms.

• 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

Understanding disease occurrence is vital for epidemiology.

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

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

• 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

Diseases vary in their severity and duration.

• 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, reducing disease spread.

Extent of Host Involvement

Infections can be localized or spread throughout the body.

• Local infection: Limited to a small area.

• Systemic 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: Initial acute infection.

• Secondary infection: Opportunistic infection after primary infection.

• Subclinical disease: No noticeable signs or symptoms.

Patterns of Disease

The progression of disease follows a predictable sequence of stages.

• 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.

Predisposing Factors

Various factors can make the body more susceptible to disease.

• Gender

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

• Climate and weather

• Lack of vaccination

• Fatigue

• Age

• Lifestyle

• Nutrition

• Chemotherapy

The Spread of Infection: Reservoirs and Transmission

Understanding sources and methods of transmission is essential for controlling infectious diseases.

• Reservoirs: Continual sources of infection.

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

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

  • Nonliving reservoirs: Soil and water.

Selected Zoonoses

Disease	Causative Agent	Reservoir	Transmission
Influenza (some types)	Influenzavirus	Swine, birds	Direct contact
Rabies	Lyssavirus	Bats, skunks, foxes, dogs, raccoons	Direct contact (bite)
Malaria	Plasmodium spp.	Monkeys	Anopheles mosquito bite
Ringworm	Trichophyton, Microsporum, Epidermophyton	Domestic mammals	Direct contact; fomites
Tapeworm (pork)	Taenia solium	Pigs	Ingestion of undercooked pork
Lyme disease	Borrelia burgdorferi	Field mice	Tick bites
Plague	Yersinia pestis	Rodents	Flea bites

Methods of Disease Transmission

Diseases can be transmitted by contact, vehicles, or vectors.

• Contact transmission: Direct (physical contact), indirect (fomites), droplet (airborne droplets <1 meter).

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

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

Representative Arthropod Vectors and Diseases

Disease	Causative Agent	Arthropod Vector
Malaria	Plasmodium spp.	Anopheles (mosquito)
African trypanosomiasis	Trypanosoma brucei	Glossina (tsetse fly)
Chagas' disease	Trypanosoma cruzi	Triatoma (kissing bug)
Lyme disease	Borrelia burgdorferi	Ixodes spp. (tick)
Plague	Yersinia pestis	Xenopsylla cheopis (rat flea)

Healthcare-Associated Infections (HAIs)

HAIs, also known as nosocomial infections, are acquired during treatment in healthcare facilities and are a major concern for patient safety.

• Affect 1 in 25 hospital patients; over 2 million infections and 70,000 deaths annually in the U.S.

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

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

Microorganisms Involved in Healthcare-Associated Infections

Microorganism	Most Common Infection Type	Percentage of Total Infections	Percentage Resistant to Antibiotics
Staphylococcus aureus	Surgical wound	16%	55%
Clostridium difficile	Diarrhea after abdominal surgery	15%	Not reported
Enterococcus spp.	Bloodstream	14%	83%
Escherichia coli	Urinary tract infections	12%	20%
Candida spp. (fungus)	Urinary tract infections	9%	Not reported

Control of Healthcare-Associated Infections

Preventing HAIs involves universal precautions and reducing pathogen numbers.

• Standard precautions: Basic, minimum practices for all patients.

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

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

Emerging Infectious Diseases

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

• Contributing factors: Genetic recombination, evolution of new strains, antibiotic resistance, changes in weather, modern transportation, insect vectors, ecological disasters, animal control, public health failure, bioterrorism.

• Examples: Escherichia coli O157, avian influenza (H5N1), Vibrio cholerae O139, Zika virus, West Nile encephalitis, Lyme disease, diphtheria.

Epidemiology

Epidemiology is the study of disease occurrence and transmission in populations. Epidemiologists identify causes, develop control methods, and analyze data.

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

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

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

The Centers for Disease Control and Prevention (CDC)

The CDC collects and analyzes epidemiological information in the U.S. and publishes the 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 period.

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