Principles of Disease, Epidemiology, and Viruses: Study Guide

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Principles of Disease and Epidemiology

Key Definitions in Pathology and Disease

Understanding the terminology of pathology and epidemiology is essential for studying infectious diseases and their impact on populations.

Pathology: The scientific study of disease, including its causes, development, and effects on the body.
Etiology: The study of the cause of a disease.
Pathogenesis: The mechanism by which a disease develops in the body.
Infection: The invasion and multiplication of pathogenic microorganisms in the body.
Disease: An abnormal state in which the body is not functioning normally due to infection or other factors.
Symbiosis: The relationship between two organisms living together.
Commensalism: A symbiotic relationship where one organism benefits and the other is unaffected.
Mutualism: Both organisms benefit from the relationship.
Parasitism: One organism benefits at the expense of the other.
Symptoms: Subjective changes in body function felt by the patient (e.g., pain, fatigue).
Signs: Objective changes that can be observed or measured (e.g., fever, rash).
Syndrome: A specific group of signs and symptoms that accompany a disease.
Communicable disease: A disease that can be spread from one host to another.
Contagious diseases: Diseases that are easily spread from one person to another.
Noncommunicable disease: A disease not spread from one host to another.
Incidence: The number of new cases of a disease in a population during a specific time period.
Prevalence: The total number of cases (new and existing) in a population at a given time.
Sporadic disease: Disease that occurs occasionally in a population.
Endemic disease: Disease constantly present in a population.
Epidemic disease: Disease acquired by many people in a given area in a short time.
Pandemic disease: An epidemic that occurs worldwide.
Acute disease: Symptoms develop rapidly but last a short time.
Chronic disease: Disease develops slowly and persists over a long period.
Subacute disease: Symptoms are between acute and chronic.
Latent disease: Disease with a period of no symptoms when the pathogen is inactive.
Herd immunity: Immunity in most of a population, reducing disease spread.
Local infection: Pathogens are limited to a small area of the body.
Systemic (generalized) infection: Infection spreads throughout the body via blood or lymph.
Focal infection: Infection that begins as a local infection but spreads to other tissues.
Sepsis: Toxic inflammatory condition from the spread of microbes or their toxins.
Bacteremia: Presence of bacteria in the blood.
Septicemia: Growth of bacteria in the blood (blood poisoning).
Toxemia: Presence of toxins in the blood.
Viremia: Presence of viruses in the blood.
Primary infection: Acute infection causing the initial illness.
Secondary infection: Opportunistic infection after a primary infection.
Subclinical infection (inapparent infection): Infection with no noticeable signs or symptoms.
Fomite: Nonliving object that can spread infection (e.g., doorknob, towel).
Epidemiology: The study of when and where diseases occur and how they are transmitted.
Descriptive epidemiology: Collection and analysis of data regarding occurrence of disease.
Analytical epidemiology: Comparison of a diseased group and a healthy group to determine cause.
Experimental epidemiology: Involves a hypothesis and controlled experiments.
Morbidity: Incidence of a specific notifiable disease.
Mortality: Deaths from notifiable diseases.
Notifiable infectious diseases: Diseases for which physicians are required by law to report cases.
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 period.

Normal and Transient Microbiota

The human body hosts a variety of microorganisms, some of which are permanent residents (normal microbiota), while others are present temporarily (transient microbiota).

Normal microbiota: Microorganisms that permanently colonize the host without causing disease under normal conditions.
Transient microbiota: Microorganisms that are present in the body for a short time without causing disease.
Opportunistic pathogens: Normal or transient microbiota that can cause disease under certain conditions (e.g., weakened immune system).
Microbial antagonism: Competition between microbes; normal microbiota can prevent the growth of harmful microbes.

Nosocomial Infections (Healthcare-Associated Infections, HAIs)

Nosocomial infections are acquired in hospitals or healthcare facilities. They are a significant concern due to increased patient vulnerability and the presence of antibiotic-resistant organisms.

Acquisition: Through direct contact with staff, other patients, or contaminated surfaces (fomites).
Control: Hand hygiene, sterilization, isolation procedures, and surveillance.

Koch’s Postulates

Koch’s postulates are criteria used to establish a 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 cultured pathogen must cause the disease when inoculated into a healthy, susceptible host.
The pathogen must be re-isolated from the inoculated host and identified as being identical to the original organism.

Exceptions: Some pathogens cannot be cultured in the lab, and some diseases are caused by multiple organisms or only cause disease in humans.

Stages in the Development of Disease

Diseases typically progress through several stages, each with characteristic features.

Incubation period: Interval between initial infection and first appearance of symptoms.
Prodromal period: Short period with early, mild symptoms.
Period of illness: Disease is most severe; signs and symptoms are evident.
Period of decline: Signs and symptoms subside.
Period of convalescence: Body returns to its pre-diseased state; recovery occurs.

Example: The flu typically has a short incubation period, followed by a prodromal period with mild symptoms, then a period of illness with fever and aches, and finally recovery.

Reservoirs and Transmission of Infection

Reservoirs are sources of infectious agents, and transmission refers to how pathogens spread.

Reservoirs: Human (carriers), animal (zoonoses), and nonliving (soil, water).
Transmission methods:
- Direct contact: Person-to-person transmission.
- Indirect contact: Via fomites.
- Droplet transmission: Coughing, sneezing.
- Vehicle transmission: Through water, food, or air.
- Vector transmission: Via insects (mechanical or biological).

Emergence of Infectious Diseases

Emerging infectious diseases are those that are new or increasing in incidence. Factors include microbial adaptation, human behavior, environmental changes, and global travel.

Predisposing Factors for Disease

Predisposing factors make the body more susceptible to disease, such as age, genetics, lifestyle, and pre-existing conditions.

Viruses: Structure, Multiplication, and Comparison with Bacteria

General Characteristics of Viruses

Viruses are acellular infectious agents that require a host cell to replicate.

Contain either DNA or RNA, not both.
Surrounded by a protein coat (capsid); some have an envelope.
Obligate intracellular parasites.

Comparison: Bacteria vs. Viruses

Feature	Bacteria	Viruses
Cell type	Prokaryotic	Acellular
Genetic material	DNA and RNA	DNA or RNA
Cell wall	Usually present	Absent
Reproduction	Binary fission	Host cell machinery
Antibiotic sensitivity	Yes	No
Size	1–10 µm	20–300 nm
Living?	Yes	No (obligate parasite)

Structural Components of a Virus

Nucleic acid: DNA or RNA (single or double-stranded).
Capsid: Protein coat surrounding the nucleic acid.
Envelope: Lipid membrane (in some viruses) derived from host cell membrane.
Spikes: Glycoprotein projections for attachment to host cells.

Plaques in Relation to Viruses

Plaques are clear zones on a layer of host cells where viruses have lysed the cells. Used to quantify viruses in a sample.

Lytic and Lysogenic Cycles of Bacteriophages

Lytic cycle: Virus replicates and lyses the host cell, releasing new virions.
Lysogenic cycle: Viral DNA integrates into the host genome as a prophage and replicates with the host cell without causing immediate lysis.
Purpose: Multiplication and survival of the virus.

Prophage

A prophage is viral DNA that has been integrated into the bacterial host genome during the lysogenic cycle.

Multiplication of Animal Viruses

Attachment: Virus binds to host cell receptors.
Entry: Virus enters the cell by endocytosis or fusion.
Uncoating: Viral nucleic acid is released from the capsid.
Biosynthesis: Viral components are produced using host machinery.
Maturation: Viral components are assembled into new virions.
Release: New virions exit the cell by lysis or budding.

Latent Viruses

Latent viruses remain in the host without causing symptoms for long periods and can reactivate later (e.g., herpes simplex virus).

Morbidity and Mortality Weekly Report (MMWR)

The MMWR is a publication by the CDC that provides data on notifiable diseases, outbreaks, and public health trends in the United States.

Formulas

Incidence rate:
Prevalence rate:
Morbidity rate:
Mortality rate:

Additional info:

Emerging infectious diseases may be caused by previously undetected or unknown infectious agents, known pathogens in new locations, or re-emergence of old diseases due to antimicrobial resistance.
Nosocomial infections are also called healthcare-associated infections (HAIs).