Principles of Disease and Epidemiology ~ Chp 14

Principles of Disease and Epidemiology

General Terms

This section introduces foundational concepts in microbiology related to disease and epidemiology. Understanding these terms is essential for studying how diseases originate, spread, and are controlled.

• Pathogen: A microorganism that can cause disease in its host.

• Etiology: The study of the cause or origin of a disease.

• Infection: The invasion and multiplication of microorganisms in body tissues, which may lead to disease.

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

Importance of Normal Flora

Normal flora refers to the population of microorganisms that reside on or within the human body without causing disease under normal conditions. They play a crucial role in maintaining health and preventing infection.

• Microbial antagonism (competitive exclusion): The process by which normal flora inhibit the growth of pathogenic microbes by:

  • Competing for nutrients

  • Competing for attachment sites on host tissues

  • Producing antimicrobial substances

• Probiotics: Live microbes that, when administered or ingested, can become part of the normal flora and promote health by antagonizing unwanted microbes.

• Disturbances of normal flora: Disruption can lead to colonization or overgrowth by pathogens, resulting in superinfection. This can be mitigated by restoring normal flora through probiotics.

Example: Administration of Lactobacillus species as probiotics to restore gut flora after antibiotic treatment.

Symbiosis

Symbiosis describes the relationship between two different organisms living in close association. In microbiology, these relationships can affect health and disease.

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

• Mutualism: Both organisms benefit.

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

Example: Escherichia coli in the human gut (mutualism: helps digest food and produces vitamins).

Opportunistic Flora

Opportunistic flora are microorganisms that do not normally cause disease but can become pathogenic under certain conditions, such as when the host's immune system is compromised or normal flora is disturbed.

• Key Point: Opportunistic infections often occur after antibiotic use or in immunocompromised individuals.

Origins of Epidemiology

Epidemiology is the study of how diseases spread and can be controlled within populations. Historical events have shaped modern epidemiological practices.

• Ignaz Semmelweis (1840s): Instituted hand washing in obstetrical clinics, significantly reducing puerperal fever rates.

• John Snow (1854): Mapped cholera cases in London, identified contaminated water as the source, and ended the epidemic by removing the Broad St. pump handle.

Reservoirs of Infection

Reservoirs are sources where infectious agents live, grow, and multiply. Understanding reservoirs is key to controlling disease spread.

• Human: People with active infections or carriers.

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

• Non-living: Soil, water, and inanimate objects can harbor pathogens.

Transmission of Disease

Transmission refers to the mechanisms by which pathogens are spread from one host to another. There are several main modes of transmission:

• Contact Transmission:

  • Direct: Person-to-person physical contact.

  • Indirect: Transmission via a fomite (non-living object, e.g., towel, syringe).

  • Droplet: Spread via respiratory droplets (e.g., sneezing, coughing) within approximately 6 feet.

• Vehicle Transmission: Pathogens transmitted via a medium such as water, food, air, or intravenous fluids.

• Vector Transmission: Arthropods (mostly insects) transmit pathogens.

  • Mechanical: Passive transport (e.g., fly carries microbes on its body).

  • Biological: Pathogen multiplies in the vector and is transmitted to the next host (e.g., Lyme disease, malaria, West Nile virus).

Health Care Associated Infections (Nosocomial)

Nosocomial infections are acquired in healthcare settings, such as hospitals. They are a significant concern due to the vulnerability of patients and the presence of many pathogens.

• Key Point: Prevention includes strict hygiene practices, such as hand washing and sterilization of equipment.

• Example: Hospital-acquired pneumonia, surgical site infections.

Mode of Transmission	Description	Example
Contact (Direct)	Physical contact between individuals	Touching, kissing
Contact (Indirect)	Via fomites (non-living objects)	Shared towels, syringes
Droplet	Respiratory droplets within 6 feet	Coughing, sneezing
Vehicle	Transmission via water, food, air, fluids	Contaminated water, foodborne illness
Vector (Mechanical)	Passive transport by arthropods	Fly lands on food
Vector (Biological)	Pathogen multiplies in vector	Malaria via mosquito

Additional info: The consensus for droplet transmission distance has shifted from 3 feet to 6 feet based on recent studies.