Principles of Disease, Epidemiology, and Viruses: Study Guide

Principles of Disease and Epidemiology

Key Definitions in Pathology and Epidemiology

Understanding the terminology is essential for grasping the concepts of disease, infection, and epidemiology. Below are definitions and explanations of important terms:

• 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 manner in which a disease develops.

• Infection: The invasion and multiplication of microorganisms in body tissues.

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

• Symbiosis: A close association between two organisms.

• 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 felt by the patient (e.g., pain, fatigue).

• Signs: Objective changes that can be observed or measured (e.g., fever, rash).

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

• Communicable disease: A disease that can be transmitted from one person to another.

• Contagious diseases: Diseases that are easily spread from person to person.

• Noncommunicable disease: A disease not transmitted from one person 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 of a disease in a population at a given time.

• Sporadic disease: Disease that occurs only occasionally.

• 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: Disease with rapid onset and short duration.

• Chronic disease: Disease that develops slowly and lasts a long time.

• Subacute disease: Disease with symptoms between acute and chronic.

• Latent disease: Disease in which the pathogen remains inactive for a time but can reactivate.

• Herd immunity: Resistance of a population to infection due to immunity of a high proportion of individuals.

• Local infection: Infection confined to a small area of the body.

• Systemic (generalized) infection: Infection spread throughout the body.

• Focal infection: Infection that starts in one area and spreads to another.

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

• Bacteremia: Presence of bacteria in the blood.

• Septicemia: Growth of bacteria in the blood.

• Toxemia: Presence of toxins in the blood.

• Viremia: Presence of viruses in the blood.

• Primary infection: Initial infection in a healthy person.

• Secondary infection: Infection caused by an opportunistic pathogen after a primary infection.

• Subclinical infection: Infection with no noticeable symptoms.

• Fomite: Nonliving object that can transmit infectious agents (e.g., doorknobs).

• Epidemiology: The study of when and where diseases occur and how they are transmitted.

• Descriptive epidemiology: Collecting and analyzing data about disease occurrence.

• Analytical epidemiology: Comparing groups to determine the cause of a disease.

• Experimental epidemiology: Using controlled experiments to study disease.

• Morbidity: The incidence of disease.

• Mortality: The incidence of death.

• Notifiable infectious diseases: Diseases that must be reported to public health authorities.

• Morbidity rate: Number of people affected by a disease per unit population.

• Mortality rate: Number of deaths from a disease per unit population.

Transient vs. Normal Microbiota

The human body hosts a variety of microorganisms, some of which are permanent residents, while others are temporary.

• Normal microbiota: Microorganisms that are always present in a healthy host.

• Transient microbiota: Microorganisms that are present for a short period and then disappear.

• Opportunistic pathogens: Normal or transient microbiota that can cause disease under certain conditions (e.g., weakened immunity).

• Microbial antagonism: Normal microbiota inhibit the growth of harmful microbes by competing for nutrients, producing substances, or altering the environment.

Nosocomial Infections (Healthcare-Associated Infections, HAIs)

Nosocomial infections are acquired in healthcare settings and pose significant risks to patients.

• Acquisition: Through direct contact, contaminated equipment, or airborne transmission.

• Control: Hand hygiene, sterilization, isolation procedures, and surveillance.

• Common pathogens: Staphylococcus aureus, Pseudomonas aeruginosa, Clostridioides difficile.

Koch’s Postulates and Exceptions

Koch’s postulates are criteria used to establish a causal 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 and grown in pure culture.

3. The cultured pathogen must cause the disease when inoculated into a healthy host.

4. The pathogen must be re-isolated from the inoculated host.

• Exceptions: Some pathogens cannot be cultured, some diseases are caused by multiple organisms, and some pathogens cause different diseases.

Stages in the Development of Disease

The progression of disease typically follows several stages:

• Incubation period: Time between initial infection and appearance of symptoms.

• Prodromal period: Early, mild symptoms.

• Period of illness: Most severe symptoms.

• Period of decline: Symptoms subside.

• Period of convalescence: Recovery and return to normal health.

Example: The flu follows these stages, with fever and malaise during the period of illness.

Reservoirs of Infection

Reservoirs are sources of infectious agents.

• Human reservoirs: People with active disease or carriers.

• Animal reservoirs: Animals that harbor pathogens (zoonoses).

• Nonliving reservoirs: Soil, water, and food.

Methods of Disease Transmission

Diseases can be transmitted in several ways:

• Direct contact: Person-to-person transmission.

• Indirect contact: Via fomites.

• Droplet transmission: Coughing, sneezing.

• Vehicle transmission: Through water, food, or air.

• Vector transmission: By insects (mechanical or biological).

Emergence of Infectious Diseases

New or increasing diseases are called emerging infectious diseases.

• Factors: Genetic changes, environmental changes, increased human contact, antibiotic resistance.

• Example: COVID-19, caused by a novel coronavirus.

Predisposing Factors for Disease

Factors that make individuals more susceptible to disease include:

• Age

• Gender

• Genetic background

• Climate and weather

• Nutrition

• Pre-existing conditions

Viruses: Structure, Multiplication, and Comparison with Bacteria

General Characteristics of Viruses

Viruses are acellular infectious agents that require a host cell for replication.

• Structure: Consist of nucleic acid (DNA or RNA) surrounded by a protein coat (capsid).

• Obligate intracellular parasites: Cannot reproduce outside a host cell.

• Size: Much smaller than bacteria.

Comparison Between Bacteria and Viruses

Feature	Bacteria	Viruses
Cellular structure	Prokaryotic cells	Acellular
Genetic material	DNA	DNA or RNA
Reproduction	Binary fission	Host-dependent
Metabolism	Present	Absent
Response to antibiotics	Usually sensitive	Insensitive
Size	1-10 µm	20-300 nm

Structural Components of a Virus

• Capsid: Protein coat surrounding the nucleic acid.

• Envelope: Lipid membrane present in some viruses.

• Spikes: Glycoproteins for attachment to host cells.

Plaques in Relation to Viruses

Plaques are clear zones on a bacterial lawn caused by viral lysis of host cells, used to quantify viruses.

Lytic and Lysogenic Cycles of Bacteriophages

• Lytic cycle: Virus infects host, replicates, and lyses the cell, releasing new viruses.

• Lysogenic cycle: Viral DNA integrates into host genome as a prophage, replicates with host, can later enter lytic cycle.

Purpose: Multiplication and spread of viral particles.

Multiplication of Animal Viruses

• Attachment

• Entry (by endocytosis or fusion)

• Uncoating

• Biosynthesis

• Assembly

• Release (by budding or lysis)

Latent Viruses

Viruses that remain dormant in host cells and can reactivate later (e.g., herpes simplex virus).

Morbidity and Mortality Weekly Report (MMWR)

Published by the CDC, the MMWR provides data on notifiable diseases, morbidity, and mortality rates in the United States.

Signs vs. Symptoms of Disease

Type	Description	Example
Signs	Objective, measurable	Fever, rash
Symptoms	Subjective, felt by patient	Pain, fatigue

Formulas for Epidemiology

• Incidence rate:

• $\text{Incidence rate} = \frac{\text{Number of new cases}}{\text{Population at risk}} \times 100$

• Prevalence rate:

• $\text{Prevalence rate} = \frac{\text{Total number of cases}}{\text{Population}} \times 100$

• Morbidity rate:

• $\text{Morbidity rate} = \frac{\text{Number of people affected}}{\text{Total population}} \times 100$

• Mortality rate:

• $\text{Mortality rate} = \frac{\text{Number of deaths}}{\text{Total population}} \times 100$

Additional info: Academic context and expanded explanations were added to ensure completeness and clarity for exam preparation.