A 2.25-g sample of scandium metal is reacted with excess hydrochloric acid to produce 0.1502 g hydrogen gas. What is the formula of the scandium chloride produced in the reaction?
Epidemiology Lesson 2 Concepts of Disease Occurrence A critical premise of epidemiology is that disease and other health events do not occur randomly in a population, but are more likely to occur in some members of the population than others because of risk factors that may be distributed randomly in the population Causation The simplest model of disease causation is the Epidemiologic Triad or Triangle (traditional model for infectious disease) o In this model, disease results from the interaction between the Agent and the susceptible Host in an Environment that supports transmission of the agent forms a source to that host o Agent: an infection microorganism, pathogen, chemical contaminant or physical force A variety of factors influence whether exposure to an organisms will result in disease including the organism’s pathologincity (ability to cause disease) and dose o Host: the human who can get the disease (Not the person who already has the disease) Opportunities for exposure are often influenced by behaviors such as sexual practice, hygiene, and other personal choices as well as by age and sex Susceptibility and response to an agent are influenced by factors such as genetic compositions, nutritional and immunologic status, anatomic, structure, presence of disease or medications and psychological makeup o Environment: extrinsic factors that affect the agent and the opportunity for exposure Include physical factors, biological factors and socioeconomic factors Epidemiology Lesson 2 1 An important job as a Epidemiologist to find the connections between the 3 factors and properly handling them in order to prevent disease from spreading Natural History of Disease The progression of a disease process in an individual over time in the absence of treatment Latency Period: With chronic diseases it is the period between the exposure and sign of symptoms Icubation Period: With infectious diseases it is the period between the exposure and sign of symptoms o Epidemiologist use a epidemic curve or histogram to find the peak of when people began to show symptoms and then begin to trace back with the incubation period for that particular disease to find the date which is most likely when they all got exposed and could also help to find if there is a common source of the disease. If the epidemic curve or histogram has 2 peaks then the time distance between the 2 peaks is the average incubation period for the disease 2 possible causes of this to happen is Propagated: the disease is being spread from person to person Epidemiology Lesson 2 2 Common Source (Continuous or intermittent): the common source of the disease is still out there and a great deal of the population is being exposed to it around the same time. Spectrum of Disease In some disease process may result in illness that ranges from mild to severe called the spectrum of disease o Infectivity: For an infectious agent it refers to the proportion of exposed persons who become infected (The ability of the thing to get in) o Pathogeniaty: refers to the proportion of infected individuals who develop clinically apparent disease (The ability of the thing, after it gets in, to cause disease in the body) o Virulence: refers to the proportion of clinically apparent cases that are severe or fatal Carriers are persons who are infectious but have subclinical disease, or who appear to have recovered from their clinical illness but remain infectious or never exhibited symptoms Component Cause and Casual Pies (Rothman in 1976) Because the agent-host-environment model did not work well for many non-infectious diseases, several other models that attempt to account for the multifactorial mature of causation have been proposed o An individual factor that contributes to cause disease is shown as a piece of pie, and often all the pieces of a pie fall into place, and the pie is complete and disease occurs Component Causes: the individual factor Sufficient Causes: the complete pie (casual pathway) o A disease may have more than one sufficient cause, with each sufficient cause being composed of several component causes that may or may not over lap Necessary Cause: a component that appears in every pie or pathway A single component cause is rarely a sufficient cause by itself Chain of Infection Epidemiology Lesson 2 3 Reservoir: the habitat in which the infectious agent normally lives, grows and multiplies (humans, animals and the environment) Portal of Exit: the path by which a pathogen leaves its host Modes of Transmissions: how an infections agent may be transmitted from its natural reservoir to a susceptible host o Direct Transmissions: An infectious agent is transferred to a susceptible host by direct contact or droplet spread Direct Contact: occurs through skin to skin contact, kissing, sexual intercourse or contact with soil or vegetation harboring infectious organisms Droplet Spread: spray with relatively large, short- range aerosols produced by sneezing, coughing, or even talking o Indirect Transmission: the transfer of an infectious agent from a reservoir to a host by suspended air particles, inanimate objects (vehicles), or animate intermediaries (vectors). Airborne: transmission occurs when infectious agents are carried by dust or nuclei suspended in air include food, water, biologic products (blood), and fomites (inanimate objects) Vectors: such as mosquitoes, fleas and ticks may carry an infectious agent through purely mechanical means or may support growth or change in agent. Portal of Entry: the manner in which a pathogen enters a susceptible host o It must provide access to tissue in which the pathogen can multiply or a toxin can act Epidemiology Lesson 2 4 o Often infectious agents use the same portal to enter a new host that they used to exit the source host Host: the final link in the chain of infection is a susceptible host o Susceptibility of a host depends on genetic or constitutional factors, specific immunity, and nonspecific factors that affect an individual’s ability to resist infection or to limit pathogenicity. The job an Epidemiologist is to break the connection between the chains. Herd Immunity The most effective way for epidemiologist to lower the effects of a disease in a community is to decrease the number of susceptible host. o With less susceptible host the disease is unable to transfer as easily By doing this those who are unable to receive vaccination for various reasons are still safe from the disease thanks to those around them who received the vaccination and are thus unable to pass the disease along to them. o Ex: The measles infectivity rate is so high that the population has to be at least 94% vaccination in order for herd immunity to properly work. Epidemic Disease Occurrence The amount of a particular disease that is usually present in a community is referred to as the baseline or Endemic Level of the disease o The level is not necessarily the desired level, which may in fact be 0, but rather the observed level, without intervention and assuming the pool of susceptible host will remain the same it will stay at this level indefinitely Sporadic: a disease that occurs infrequently and irregularly Epidemiology Lesson 2 5 Endemic: the constant presence and /or prevalence of a disease or infectious agent in a population within a geographic area Hyperendemic: persistent, high level of disease o Epidemics: an increase, often sudden in the number of cases of a disease above what is normally expected in that population in that area o Outbreak: carries the same definition of epidemic, but is often used for a more limited geographic area (Usually from a single source and usually doesn’t spread too far outside of the area) o Cluster: an aggregation of cases grouped in place and time that are suspected to be greater than the number expected, even though the expected number may not be known ( Usually not infectious diseases but chronic) o Pandemics: epidemic that has spread over 2 or more countries or continents, usually affecting a large number of people. Epidemic Patterns Epidemics can be classified according to their manner of spread though a population o Common-Source Outbreak: a group of persons are all exposed to an infectious agent or a toxin from the same source Point-Source Outbreak: the group is exposed over a relatively brief period, so that everyone who becomes ill does do within 1 incubation period Continuous Common-Source Outbreak: the range of exposures and range of incubation period tend to flatten and widen the peaks of the epidemic curve Intermittent Common-Source Outbreak: often has a pattern reflecting the intermittent mature of the exposure o Propagated Outbreak: results form transmission is by direct person to another o Mixed Epidemics: the pattern of a common-source outbreak followed by secondary person to person spread Some epidemics are neither common-source in its usual sense nor propagated from person to person Necessary/Sufficient A necessary cause of disease is something that MUST be present in order for a disease to occur A sufficient cause of disease is something that can cause disease WITHOUT ANOTHER FACTOR’S ASSISTANCE Epidemiology Lesson 2 6 o Necessary AND Sufficient (Factor A)=Factor ADisease The factor is both necessary to cause disease, and sufficient ALONE to cause the disease o Necessary BUT NOT Sufficient (Factor A)=Factor B OR Factor C + Factor ADisease The factor is necessary to cause the illness, but can not cause the illness without help o Sufficient BUT NOT Necessary (Factor A)=Factor A OR Factor B OR Factor C Disease The factor is sufficient to cause disease (it can cause the disease all by itself), but the single factor alone is not necessary… other things can also cause disease by themselves o NOT Necessary NOR Sufficient (Factor A) =Factor A + Factor B OR Factor C + Factor D Disease Probably most accurately represents clinical medicine Epidemiology Lesson 2 7