DP - Disease Transmission Lesson
Disease Transmission
Have you ever noticed how a cold seems to travel from one person to another? First, your friend gets sick. Then, you get sick. Next, your teacher gets sick, followed by one or both of your parents. Soon, half the class is suffering with the same symptoms. When a disease outbreak affects many people, it is called an epidemic. The word "epidemic" comes from the Greek words meaning "common among the people". A cluster is a group of cases in a specific time and place that may or may not be greater than the expected rate. Often the aim of investigating clusters is to determine the baseline rate of disease for that time and place. Another term you will come across is "endemic", meaning a high background rate of disease. When this common sickness grows to global proportions it is called a "pandemic".
In the 16th century, most people believed that disease was spread by foul air. There were no microbes, germs, or viruses - just air that had lost its ability to keep people healthy. So strong was this belief, that the Italian phrase for bad air, "malaria" was used to identify a common blood disease.
In 1546, a revolution occurred. In a work entitled "On Contagion", Girolamo Francastoro proposed controversial beliefs on the spread of disease. He suggested that illness could be spread by a disease agent. Fracastoro argued that air transferred unseen agents from the infected to uninfected individuals. The infection progressed into a sickness that was accompanied by disease symptoms.
Laboratory data backing Francastoro's work was slow to appear. It took until 1870 for Louis Pasteur to fully present his convictions on microbes and disease. Pasteur's beliefs, presented in his Germ Theory of Disease, implicated microbes as the cause of infectious disease.
Material adapted from Biologycorner.com
Modes of Transmission
When an agent leaves the source, a means of carrying it to the host, or mode of transmission is needed. This can happen in a number of ways, some of which are direct and some indirect. Direct transmission includes contact with soil or plants as well as contact between people or between animals and people. Examples of direct transmission include mucous membrane to mucous membrane contact such as in sexual intercourse, skin to skin, and ingestion of infected food. In indirect transmission, the agent can be airborne, vector borne, or vehicle borne. In airborne transmission, the agent is carried from the source to the host suspended in air particles. This may occur when a person sneezes, coughs, or talks into the air that is breathed in by a nearby person. Vector-borne diseases are transmitted indirectly by a live carrier, usually an arthropod, such as mosquitos, fleas, or ticks. Vehicle-borne diseases are carried by inanimate objects, such as food or water, blood, or items like handkerchiefs, bedding, and surgical instruments.
The following chart displays the different modes of disease transmissions and ensuing diseases.
| Mode of Transmission | Diseases |
|---|---|
| Direct | |
| Person to Person | AIDS |
| Animal to Person | Rabies |
| Indirect | |
| Airborne | Tuberculosis |
| Vector-borne | Malaria |
| Vehicle-borne | Cholera |
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