Chapter 7: Communicable diseases

Endemic

Regularly occurring within an area or community

Epidemic

A widespread occurrence of an infectious disease in a community at a particular time

Pandemic

A widespread occurrence of an infectious disease over a whole country or the world at a particular time

Epidemiological Transition

The shift in population disease patterns from infectious to chronic and degenerative diseases as societies develop.

Routes of Transmission

The pathways through which infectious agents move from one host to another, including direct contact, respiratory droplets or airborne particles, fecal‑oral spread, contaminated surfaces, vectors like mosquitoes, and vertical transmission from parent to offspring.

Direct

Skin-to-skin contact, over a short distance, or perineal

Direct Contact

skin‑to‑skin contact, kissing, sexual intercourse, mother‑to‑child transfer

Droplet

(typically >5 µm) from coughing or sneezing reach the eyes, nose, or mouth within close range.

Indirect

Airborne over long distances, on surfaces, in water and food, or through an intermediary like a vector.

Airborne

The spread of infectious diseases through tiny respiratory particles that remain suspended in the air for extended periods, allowing pathogens such as measles, tuberculosis, and influenza to travel beyond close contact and infect people at a distance.

Vector-borne

The spread of infectious pathogens through living organisms—mainly mosquitoes, ticks, fleas, and flies—that pick up a pathogen from an infected host and deliver it to a new host during feeding. This process can involve biological development of the pathogen inside the vector, as seen in malaria, or simple mechanical carriage, as with houseflies.

Vehicle-borne

Mode of infectious disease spread in which pathogens are carried to a host via contaminated inanimate objects (fomites) such as food, water, utensils, medical instruments, bedding, or clothing.

R0 (Rnaught)

Is a statistic that measures the average number of infections produced by an average infected individual exposed to an otherwise susceptible population. Can be influenced by external forces.

Epidemiological Triad

A framework for thinking about disease transmission but also for intervention

1. host

2. agent

3. environment

Chain of Infection

6-step cycle showing how infection spreads:

1. infectious agent

2. resivoir

3. portal of exit

4. mode of transmission

5. portal of entry

6. susceptible host

Vaccines

a substance used to stimulate immunity to a particular infectious disease or pathogen, typically prepared from an inactivated or weakened form of the causative agent or from its constituents or products

Live attenuated vaccine

Contains a weakened form of a living virus or bacterium that can still replicate but does not usually cause disease in healthy people. Because it closely mimics a natural infection, it often produces a strong and long-lasting immune response

Inactivated Vaccine

contains a virus or bacterium that has been killed or inactivated, so it cannot reproduce or cause the disease.

Toxoid Vaccine

contains an inactivated (detoxified) bacterial toxin. The toxin can no longer cause disease, but it still stimulates the immune system to produce antibodies against the toxin.

mRNA Vaccine

Contains messenger RNA (mRNA) that provides instructions for cells to make a harmless piece of a pathogen (usually a protein). The immune system recognizes this protein as foreign and builds immunity against it.

Herd Immunity

Occurs when a large portion of a population becomes immune to a disease, making it difficult for the pathogen to spread from person to person.

Elimination

means you have expelled the disease from a given region.

Eradication

means there is no known circulation of the disease in the world.

Characteristics that make disease eradication possible

6 characteristics:

1. No animal reservoir

2. Effective prevention method available

3. Accurate diagnosis

4. Limited ability to spread

5. Long-lasting immunity

6. No asymptomatic carriers (or very few)