14 Principles of Disease and Epidemiology: Study Notes

Principles of Disease and Epidemiology

Pathology, Etiology, Infection, and Disease

Understanding the basic concepts of disease is fundamental in microbiology. These terms describe the processes by which diseases develop and affect the human body.

• Pathology: The scientific study of disease, focusing on the causes, development, and effects of diseases on the body.

• Etiology: The cause or origin of a disease.

• Pathogenesis: The mechanism by which a 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 reside on and within the human body. These microbial communities play essential roles in health and disease.

• Acquisition: The microbiome begins to establish in utero and expands through exposure to food, people, and pets after birth.

• Normal Microbiota: Microbes that permanently colonize the host without causing disease under normal conditions.

• Transient Microbiota: Microbes that are present temporarily and may disappear after days, weeks, or months.

• Factors Affecting Distribution: Nutrients, physical and chemical factors, host defenses, and mechanical factors determine the composition and distribution of microbiota.

Representative Normal Microbiota by Body Region

The following table summarizes the principal microbial inhabitants of various body regions and their significance:

Region	Principal Components	Comments
Skin	Propionibacterium, Staphylococcus, Corynebacterium, Micrococcus, Acinetobacter, Brevibacterium, Candida, Malassezia	Secretions inhibit many microbes; low pH and moisture content limit colonization.
Eyes (Conjunctiva)	Staphylococcus epidermidis, S. aureus, diphtheroids, Propionibacterium, Corynebacterium, streptococci, Micrococcus	Tears and blinking remove microbes; similar to skin microbiota.
Nose and Throat	Staphylococcus aureus, S. epidermidis, diphtheroids, Streptococcus pneumoniae, Haemophilus, Neisseria	Microbial antagonism reduces pathogen colonization; mucus and cilia remove microbes.
Mouth	Streptococcus, Lactobacillus, Actinomyces, Bacteroides, Veillonella, Neisseria, Haemophilus, Fusobacterium, Treponema, Staphylococcus, Corynebacterium, Candida	Moisture and food support diverse populations; saliva contains antimicrobials.
Large Intestine	Escherichia coli, Bacteroides, Fusobacterium, Lactobacillus, Enterococcus, Bifidobacterium, Enterobacter, Citrobacter, Proteus, Klebsiella, Candida	Largest numbers of microbiota; mucus and shedding prevent attachment.
Urinary and Reproductive Systems	Staphylococcus, Micrococcus, Enterococcus, Lactobacillus, Bacteroides, diphtheroids, Pseudomonas, Klebsiella, Proteus, Candida albicans, Trichomonas vaginalis	Acidic secretions and urine flow inhibit microbes; periodic shedding prevents attachment.

Microbial Antagonism and Symbiosis

Normal microbiota protect the host through microbial antagonism, also known as competitive exclusion. Symbiotic relationships between microbes and the host can be classified as commensalism, mutualism, or parasitism.

• Microbial Antagonism: Normal microbiota compete for nutrients, produce harmful substances, and alter environmental conditions to inhibit pathogens.

• Commensalism: One organism benefits, the other is unaffected (e.g., Staphylococcus epidermidis on skin).

• Mutualism: Both organisms benefit (e.g., Escherichia coli in the large intestine synthesizes vitamins for the host).

• Parasitism: One organism benefits at the expense of the other (e.g., influenza virus in a host cell).

• Opportunistic Pathogens: Normally harmless microbes that cause disease under certain conditions, such as immunosuppression.

Koch’s Postulates and the Etiology of Infectious Diseases

Koch’s postulates provide a framework for identifying the causative agent of infectious diseases. However, there are exceptions to these postulates due to the complexity of some pathogens and ethical considerations.

• 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 animal.

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

• Exceptions: Some pathogens cause multiple diseases, only infect humans, or cannot be cultured in the laboratory.

Classifying Infectious Diseases

Diseases are classified based on their communicability, frequency, and severity. Understanding these classifications helps in epidemiological studies and public health interventions.

• Symptoms: Subjective changes felt by the patient (e.g., pain).

• Signs: Objective changes observed or measured (e.g., fever).

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

• Communicable Disease: Spread from one host to another (e.g., chickenpox).

• Contagious Disease: Easily and rapidly spread (e.g., measles).

• Noncommunicable Disease: Not spread from host to host (e.g., tetanus).

Occurrence and Frequency of Disease

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

• Prevalence: Total number of cases (new and existing) at a given time.

• Sporadic Disease: Occurs occasionally (e.g., typhoid fever).

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

• Epidemic Disease: Many people acquire in a short time (e.g., influenza).

• Pandemic Disease: Worldwide epidemic (e.g., COVID-19).

Severity or Duration of Disease

• Acute Disease: Rapid onset, short duration (e.g., influenza).

• Chronic Disease: Develops slowly, lasts long (e.g., tuberculosis).

• Subacute Disease: Intermediate between acute and chronic.

• Latent Disease: Pathogen inactive for a time, then activates (e.g., shingles).

• Herd Immunity: Immunity in most of a population, limiting outbreaks.

Extent of Host Involvement

• Local Infection: Pathogens limited to a small area (e.g., boils).

• Systemic Infection: Spread throughout the body (e.g., measles).

• Focal Infection: Local infection spreads to other parts.

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

• Septicemia: Growth of bacteria in the blood.

• Bacteremia: 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

The development of disease follows a sequence of stages:

• Incubation Period: Time 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: Recovery and return to prediseased state.

Reservoirs and Transmission of Disease

Reservoirs are continual sources of infection. Transmission can occur through various routes:

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

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

• Nonliving Reservoirs: Soil and water.

Disease	Causative Agent	Reservoir	Transmission
Rabies	Lyssavirus	Bats, dogs, etc.	Bite (direct contact)
Malaria	Plasmodium spp.	Monkeys	Mosquito bite
Anthrax	Bacillus anthracis	Livestock	Contact, air, food
Ringworm	Trichophyton spp.	Domestic mammals	Direct contact
Tapeworm	Taenia solium	Pigs	Ingestion of undercooked pork
Salmonellosis	Salmonella enterica	Poultry, reptiles	Ingestion, contact

Modes of Transmission

• Direct Contact: Person-to-person transmission.

• Indirect Contact: Via fomites (nonliving objects).

• Droplet Transmission: Airborne droplets less than 1 meter.

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

• Vector Transmission: Arthropods (e.g., mosquitoes, ticks) transmit pathogens mechanically or biologically.

Healthcare-Associated Infections (HAIs)

HAIs, or nosocomial infections, are acquired during treatment in healthcare facilities. They are a significant cause of morbidity and mortality.

• Sources: Hospital environment, compromised hosts, chain of transmission.

• Compromised Host: Individual with impaired resistance (e.g., burn patients).

• Prevention: Handwashing, disinfection, infection control committees, and universal precautions.

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

Emerging Infectious Diseases (EIDs)

EIDs are diseases that 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 misuse, changes in weather, modern transportation, ecological changes, public health failures, and bioterrorism.

• Examples: Avian influenza, Zika virus, antibiotic-resistant bacteria, Lyme disease, anthrax.

Epidemiology

Epidemiology is the study of where and when diseases occur and how they are transmitted in populations. Epidemiologists identify causes, track disease spread, and develop control methods.

• Descriptive Epidemiology: Collects data on disease occurrence (e.g., John Snow and cholera).

• Analytical Epidemiology: Analyzes data to determine probable causes (e.g., Florence Nightingale and sanitation).

• Experimental Epidemiology: Tests hypotheses through controlled experiments (e.g., Semmelweis and handwashing).

• CDC (Centers for Disease Control and Prevention): Collects and analyzes epidemiological data, 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 to public health authorities.