Principles of Disease and Epidemiology – Study Guide Notes

Principles of Disease and Epidemiology

Symbiotic Relationships Between Microorganisms and Hosts

Microorganisms interact with their hosts in various ways, forming different types of symbiotic relationships. Understanding these relationships is essential for grasping the dynamics of infectious diseases.

• Mutualism: Both organisms benefit from the relationship. Example: Escherichia coli in the human intestine synthesizes vitamin K and B vitamins for the host while obtaining nutrients.

• Commensalism: One organism benefits, and the other is unaffected. Example: Skin bacteria that live on secretions without harming the host.

• Parasitism: One organism (the parasite) benefits at the expense of the other (the host). Example: Pathogenic bacteria causing disease in humans.

Normal flora (normal microbiota) typically represent mutualism or commensalism. They protect the host by:

• Competing with pathogens for nutrients and space

• Producing substances harmful to invading microbes

• Stimulating the host’s immune system

Factors causing changes in normal flora: Antibiotic use, changes in diet, illness, or hormonal changes can disrupt the balance, leading to overgrowth of opportunistic pathogens.

Transient microbiota are microorganisms that are present temporarily and do not establish permanent residence.

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 microorganism must be found in all organisms suffering from the disease, but not in healthy organisms.

• 2. The microorganism must be isolated from a diseased organism and grown in pure culture.

• 3. The cultured microorganism should cause disease when introduced into a healthy organism.

• 4. The microorganism must be re-isolated from the inoculated, diseased experimental host and identified as being identical to the original specific causative agent.

Significance: These postulates provide a framework for identifying the cause of infectious diseases.

Exceptions: Some pathogens cannot be cultured in the laboratory, some diseases are caused by multiple organisms, and some pathogens cause different diseases in different hosts.

Key Terms in Infectious Disease

• Infection: Invasion and multiplication of microorganisms in body tissues.

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

• 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 specific group of signs and symptoms that accompany a disease.

• Bacteremia: Presence of bacteria in the blood.

• Septicemia: Growth of bacteria in the blood, often leading to sepsis.

• Sepsis: A toxic inflammatory condition arising from the spread of microbes, especially bacteria or their toxins, from a focus of infection.

• Toxemia: Presence of toxins in the blood.

• Viremia: Presence of viruses in the blood.

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

• Focal infection: Infection that began as a local infection but has spread to other tissues.

• Systemic infection: Infection spreads throughout the body via blood or lymph.

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

• Secondary infection: Infection caused by an opportunistic pathogen after the primary infection has weakened the host’s defenses.

Classification and Epidemiological Terms

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

• 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: Worldwide epidemic.

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

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

• 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 causative agent is inactive.

• Herd immunity: Immunity in most of a population, reducing the spread of disease.

• Reservoir of infection: Continual source of infection (human, animal, or nonliving).

• Zoonotic disease (zoonosis): Disease that can be transmitted from animals to humans.

• Fomites: Nonliving objects that can spread infection (e.g., doorknobs, towels).

Stages of Disease

Diseases typically progress through several stages:

• Incubation period: Interval between initial infection and first appearance of signs and symptoms (no symptoms/signs).

• Prodromal period: Short period with early, mild symptoms (some symptoms/signs).

• Period of illness: Disease is most severe; symptoms and signs are most evident.

• Period of decline: Signs and symptoms subside.

• Period of convalescence: Body returns to its pre-diseased state; recovery occurs.

Symptoms and signs are most prominent during the period of illness and may begin in the prodromal period.

Reservoirs of Infection

Reservoirs are sources where pathogens persist and provide opportunities for transmission.

• Human reservoirs: People with active infections or carriers (e.g., Typhoid Mary).

• Animal reservoirs: Wild and domestic animals (e.g., rabies in bats, Lyme disease in deer).

• Nonliving/environmental reservoirs: Soil, water, and food (e.g., Clostridium tetani in soil).

Zoonoses are diseases transmitted from animals to humans (e.g., rabies, influenza).

Fomites are inanimate objects that can transmit pathogens (e.g., bedding, utensils).

Transmission of Disease

Pathogens can be transmitted by various routes:

• Contact transmission:

  • Direct contact: Person-to-person physical contact (e.g., touching, kissing).

  • Indirect contact: Transmission via fomites.

  • Droplet transmission: Spread via droplets over short distances (e.g., coughing, sneezing).

• Vehicle transmission: Transmission by a medium such as food, water, or air.

• Vector transmission:

  • Mechanical transmission: Passive transport on the body of an arthropod (e.g., flies transferring pathogens on their feet).

  • Biological transmission: Pathogen reproduces in the vector and is transmitted via bites (e.g., malaria by mosquitoes).

Cross-contamination refers to the transfer of pathogens from one food or surface to another, often via hands or utensils.

Most difficult transmission to control: Airborne transmission is often the hardest to control due to the mobility and persistence of pathogens in the air.

Nosocomial (Healthcare-Associated) Infections

Nosocomial infections are acquired in hospitals or healthcare facilities.

• Factors responsible:

  • Presence of microorganisms in the hospital environment

  • Compromised host (patients with weakened immune systems)

  • Chain of transmission (staff, equipment, other patients)

• Most predominant type: Urinary tract infections are the most common nosocomial infections.

• Why they occur: High concentration of pathogens, invasive procedures, and immunocompromised patients increase risk.

• Prevention: Hand hygiene, sterilization, use of personal protective equipment, and infection control protocols.

Transmission Route	Definition	Example
Direct Contact	Physical contact between infected and susceptible host	Touching, kissing
Indirect Contact	Via fomites (inanimate objects)	Doorknobs, towels
Droplet	Short-range airborne droplets	Coughing, sneezing
Vehicle	Transmission by food, water, or air	Contaminated water
Vector (Mechanical)	Passive transport by arthropod	Flies on food
Vector (Biological)	Pathogen reproduces in vector	Malaria by mosquitoes

Example: Salmonella can be transmitted via contaminated food (vehicle transmission) or by contact with infected animals (zoonosis).

Additional info: Understanding these principles is crucial for controlling infectious diseases and preventing outbreaks in both community and healthcare settings.