Principles of Disease and Epidemiology

Key Terms and Definitions

This section introduces essential terminology and concepts for understanding the principles of disease, epidemiology, and microbial pathogenicity. Mastery of these terms is crucial for analyzing disease patterns, transmission, and host-pathogen interactions in microbiology.

• Symptoms: Subjective changes in body function felt by a patient, such as pain or fatigue.

• Signs: Objective changes that can be measured or observed, such as fever or rash.

• Communicable disease: A disease that can be spread from one host to another.

• Contagious disease: A communicable disease that is easily spread.

• Non-communicable disease: A disease not spread from one host to another.

• Sporadic disease: Disease that occurs only occasionally.

• Endemic disease: Disease constantly present in a population.

• Epidemic disease: Disease acquired by many hosts in a given area in a short time.

• Pandemic disease: Worldwide epidemic.

• Incidence: Number of people who develop a disease during a particular time period.

• Prevalence: Number of people who have a disease at a specified time.

• Acute disease: Disease in which symptoms develop rapidly but last a short time.

• Chronic disease: Disease develops slowly and persists over a long period.

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

• Latent disease: Disease with a period of no symptoms when the causative agent is inactive.

• Herd immunity: Immunity in most of a population.

• Local infection: Pathogens are limited to a small area of the body.

• Systemic infection: Infection throughout the body.

• Focal infection: Systemic infection that began as a local infection.

• Predisposing factors: Factors that make the body more susceptible to disease.

• Incubation: Interval between initial infection and first appearance of symptoms.

• Prodromal: Early, mild symptoms of disease.

• Period of illness: Disease is most severe.

• Period of decline: Signs and symptoms subside.

• Period of convalescence: Body returns to pre-diseased state.

• Reservoir of infection: Continual source of infection.

• Direct contact transmission: Requires close association between infected and susceptible host.

• Indirect contact transmission: Spread by fomites (nonliving objects).

• Fomite transmission: Transmission via inanimate objects.

• Droplet transmission: Transmission via airborne droplets.

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

• Vector transmission: Transmission by animals, especially arthropods.

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

• Mechanical transmission: Arthropod carries pathogen on feet.

• Nosocomial infections (HAI): Infections acquired in hospitals and other healthcare facilities.

• Aseptic: Free from contamination caused by harmful bacteria, viruses, or other microorganisms.

Microbial Interactions and Pathogenicity

Understanding the relationships between microbes and hosts is essential for grasping disease mechanisms and epidemiology.

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

• Mutualism: Both organisms benefit.

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

• Opportunistic pathogen: Normally harmless but can cause disease under certain conditions.

• Primary pathogen: Causes disease in healthy hosts.

• Microbial antagonism: Competition between microbes.

• Competitive exclusion: Normal microbiota prevent colonization by pathogens.

• Mortality/Morbidity: Death rate/illness rate in a population.

• Virulence: Degree of pathogenicity.

• Compromised host: Host with impaired resistance to infection.

• Primary infection: Acute infection that causes the initial illness.

• Secondary infection: Opportunistic infection after a primary infection.

• Subclinical (apparent) infection: No noticeable signs or symptoms.

• Carrier: Person who harbors a pathogen and transmits it to others.

• Microbiota: Microorganisms that are normally present in and on the human body.

• Pathogenesis: Development of disease.

• Etiology: Study of the cause of disease.

• Pathology: Study of disease.

Koch's Postulates

Koch's postulates are a set of criteria used to establish a causative 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 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.

Transmission of Disease: Table

The following table summarizes major modes of disease transmission:

Additional info:

• Understanding these terms is foundational for further study in epidemiology, infection control, and clinical microbiology.

• Nosocomial infections are also known as healthcare-associated infections (HAI).

• Herd immunity is critical for preventing outbreaks, especially in vaccine-preventable diseases.