Communicable Disease Concepts
1. communicable transmission
disease: of an agent (or its by-product)
from reservoir
to host.
In order to understand the communicable disease, we need to define the four terms underlined in the above definition. These four terms are defined below.
2. agent: a factor that must be present for a
disease to occur in a susceptible host.
Environmental health professionals are normally concerned with
physical, chemical, or biological agents. However, we also refer
to psychological and sociological agents. Examples of agents include:
pathogenic microbes, parasitic worms, and chemical poisons.
3. infection: growth of a pathogenic microbe in a host
(with or without evidence of disease)
4. pathogenicity: capable of producing disease.
If a "case" is defined as someone who has the disease, then pathogenicity is normally measured as: [# of cases[ / [# infected]
5. virulence: harmfulness of a disease.
The most commonly used measure for virulence is: case fatality rate = [# of deaths] / [total # of cases] For example, a 100% case fatality rate means that everyone who contracts the disease will die.
6. reservoir: any place where an infectious agent
depends primarily for survival. A "place" is broadly
defined and can include inanimate objects, but may
also include inside humans.
7. host: any animal (especially human) infected by an agent;
host may be diseased or
may be an intermediate host.
8. incubation time interval between exposure
period: to infectious agent and first
sign or symptom of disease.
9. carrier: a person or animal that harbors an organism
of disease without showing symptoms.
10. asymptomatic
carrier: a carrier that never shows symptoms
(also called "inapparent infection").
11. transmission: any mechanism by which a susceptible human
host is exposed to an infectious agent.
12. fomites: inanimate objects (other than food, water)
which harbor or transmit infectious organisms.
13. vector: insect or other animal that may transfer
pathogens to humans.
14. infestation:
humans, lodgment, development, and reproduction
animals: of arthropods on the surface of the
body or in clothing.
articles, harboring or sheltering animals
premises: (especially arthropods or rodents).
15. LD-50 (or LD50): Lethal dose for 50% of the exposed subjects.
A low LD50 indicates a very toxic substance, because it says that a small dose is capable of killing half the population. A different measure is LC50, which is the lethal concentration for 50% of the population. Concentration refers to measured amounts in the environment, whereas dose refers to measured amounts in the body.
16. Incidence: [# of new cases in a length of time] / [# of people exposed]
Incidence can tell us a great deal about the causality of disease. For example, if there was a release of a chemical agent in a small town followed by 80% of the town becoming sick over a period of one year, we would be very suspicious that this large number of new cases would be associated with the chemical's release.
From the standpoint of our model of communicable disease, we can divide controls into 3 basic categories. This approach may actually be more helpful in divising strategies for controlling the spread of communicable disease.
1. Control the agent:
For example, we can remove agents before their entry into air, water, and soil. If it is a chemical agent, this may involve simple changes in production processes. If it is a microbial agent, it may involve prohibiting consumption of affected foods, or use of bactericides on preparation surfaces.
2. Control the environment:
For example, we can control control vectors, or treat polluted air, water, and soil. We can also prevent access to an area.
3. Control the host:
For example, we can take steps to protect the young, the old, and the sick, each of whom may be the high risk individuals. This may involve personal hygiene, immunizations, or health education.
1. Common cold agent: rhinoviruses, coronaviruses, others reservoir: human transmission: direct contactWhy has there been no cure for the common cold? Because the common cold is not a single agent. In fact, the cause of the common cold is unidentified in roughly half of all cases! Viruses are most often involved, but bacteria may be as well. The point is that we cannot say what the agent of the common cold is, because it varies! The recent progress in treatment methods for the common cold (zinc, ehinacea, vitamin C, garlic, goldenseal, various teas, and various nose sprays) may address its symptoms, but effectiveness with all agents is the true challenge.2. Influenza: agent: influenza viruses (3 major serotypes): type A: widespread, pandemics type B: local epidemics type C: only in sporadic cases reservoir: human transmission: same as the common cold 3. Tuberculosis: agent: Mycobacterium reservoir: primarily human transmission: primarily droplet nucleiTuberculosis is spread by droplet nuclei from an infected individual. For example, if an individual sneezes or coughs, they may spread droplets (moisture that emanates from the sneeze or cough). Droplet nuclei refer to the residue that is left after the droplets have dried. The spread of tuberculosis from these droplet nuclei normally requires lengthy exposure (i.e., casual short-term exposure is less likely to transmit the disease).4. Coccidioidomycosis: agent: Coccidioides immitis (fungus) reservoir: soil transmission: airborneCoccidioides immitis lives in hot, dry, dusty environments. People contract the disease by inhaling the fungal spores that become airborne.5. Pneumonia: inflammation of the lungs with congestion (not a specific disease, but a pathological term -- i.e., symptoms) reservoir: essentially human transmission generally direct, sometimes airborne various agents: 5. pneumococcal pneumonia (Streptococcus pneumoniae) 6. mycoplasmal pneumonia (Mycoplasma pneumoniae) 7. pneumocystis pneumonia (Pneumocystis carinii) 8. chlamydial pneumonia (Chlamydia trachomatis) 9. others: various viruses, rikettsias (Q fever), Legionella, worms (nematodes), etc.
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