LESSON 1: Communicable Diseases
1-8. CYCLE OF INFECTION
Prevention and control of infection is of vital importance to the patient as well as to health care personnel. In order to provide proper care for patients with communicable diseases or infectious organisms, you should understand the components of infection and the methods to control the cycle of infection. The cycle of infection (see figure 1-1) is like a chain consisting of six links. To produce disease, each link of the infectious process must be present in a logical sequence. Removing one link in the chain will control the cycle of infection. The six links are discussed in the following paragraphs.
Figure 1-1. The cycle of infection.
a. Infectious Microorganisms (Agent). These are the pathogens that cause communicable diseases.
b. Reservoir. The reservoir (source) is the person or animal that has the disease. Sometimes a person may have a disease but is not ill. This type of person is called a carrier. The carrier known as Typhoid Mary is a classic example. She was a food worker in a restaurant who spread the disease typhoid by contaminating the foods she handled. Other examples of reservoirs are a person with a common cold, a person with malaria, a person with syphilis, a rat with plague, and a bat with rabies.
c. Mode of Escape. This refers to the route by which the infectious microorganisms escape the reservoir. For example, pathogens that cause respiratory diseases usually escape through the respiratory tract (coughing, sneezing, and so forth) Modes of escape and methods of controlling the mode (preventing the escape) are shown in figure 1-2.
Figure 1-2. Modes of escape and their control.
d. Vector. The vector is the connection between the source of the disease (reservoir) and the person who is going to catch the disease (host). The vector is sometimes referred to as the "vehicle of disease transmission." Vectors and their control are discussed in figure 1-3.
Figure 1-3. Vectors and their control
e. Mode of Entry. The mode of entry refers to the method by which the pathogens enter the person (host). For example, some pathogens are inhaled (respiratory tract). Ways of controlling modes of entry are shown in figure 1-4.
Figure 1-4. Modes of entry and their control.
f. Susceptible Host. The host is the person who gets the disease. Once the host has the disease, he becomes a reservoir for future transmission of the disease. A listing of the most susceptible persons to disease and some control measures are shown in figure 1-5.
Figure 1-5. Susceptible hosts and control measures.
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