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TOPIC 6.docx

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University of Waterloo
HLTH 101
Glenn Ward

TOPIC 6: CHANGES IN HEALTH THROUGH EMERGING AND RE-EMERGING INFECTIOUS DISEASES 2: THE EPIDEMIOLOGY OF INFECTIOUS DISEASES 1. THE EPIDEMIOLOGY OF INFECTIOUS DISEASES i. Attributing Causality of Infectious Diseases  historically, it was very difficult to attribute a particular disease to infection with a particular agent  late 1800ʼs: Robert Koch, a German scientist, formulated rules for determining if a particular microbe was the causative agent of a particular disease  Kochʼs Postulates* 1. Association: the causative agent is present in every case of the disease 2. Isolation: the causative agent can be isolated in every case of the disease and grown in pure culture 3. Causation: the isolated, cultured agent must cause the disease when it is inoculated into healthy susceptible animals 4. Re-isolation: the causative agent, when isolated from the inoculated animal, must be identical to the original agent  the rigidity of the postulates reflected the time Koch wrote them, since some scientists werenʼt completely convinced that infectious agents caused diseases  because the burden of proof lay so heavily against those trying to prove this connection, the postulates were quite strict  today, while the postulates are still useful, we accept that there are many exceptions to them: o infectious agents will not always be identified in infected humans o syphilis and many viruses cannot be not easily, if at all, grown in pure culture o Helicobactor pylori will not cause disease in most people infected with it o hepatitis B and yellow fever cannot easily be used to cause disease in animals ii. Classification of Infectious Diseases* a) sporadic  sporadic diseases are infectious diseases that occur randomly and occasionally they tend to be diseases that are not transmitted easily between humans  tetanus and rabies are examples of sporadic diseases b) endemic  endemic diseases are infectious diseases that occur at a low and relatively constant rate*  these diseases are generally not considered to be a public health threat, although they may become so under certain circumstances  the common cold and chicken pox are examples of endemic diseases in our society today  geographic factors can determine if a disease is classified as endemic or not o for example, Lyme disease can be considered to be endemic in some geographic areas and sporadic in others c) epidemic  an epidemic is the state of an infectious disease associated with increased morbidity and mortality within a specific area of the world* o epidemics are generally seen as public health problems or potential public health problems  usually epidemics were formerly sporadic or endemic (or even nonexistent) diseases  generally they can be divided into two categories: 1) common-source epidemics* o these are epidemics that arose in many individuals from a single source and are unlikely to be transmitted from person-to-person o they are notable for relatively rapid increases and decreases in incidence o food or water contamination are usually the source of common-source epidemics 2) propagated epidemics* o these are epidemics that arise through person-to-person transmission o because all of those infected do not get sick at approximately the same time, propagated epidemics are notable for relatively slower increases and decreases in incidence o chicken pox, measles, and mumps are common examples of propagated epidemics d) pandemic  these
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