PART I QUESTIONS 1. Models are analogies that allow us to clarify hypotheses—proposed explanations of relationships between causes and effects. What roles do models play in testing hypotheses? Models provide the physical testing and proof of a hypothesis by exploring the extent to which the two factors relate within the given hypothesis. It puts a theory into action, to see if the theory is correct. 2. What did the humoral model of disease propose as the cause for cholera?
The humoral model of disease said that disease was caused by an imbalance in one or more of four “humors” or fluids in the body: blood, phlegm, black bile, and yellow bile. Physicians would decide on a treatment based on what they thought was the cause of the imbalance. There were treatments that included the act of “blood-letting” where they would either cut the person, and allow them to bleed for a short time, or apply leaches to them to remove blood. 3.
What did the miasma model of disease propose as the cause for cholera? The miasma model of disease proposed that the cause for cholera was caused and spread from person to person through bad vapors or gases in the air. 4. Unlike Snow’s later work on cholera, his research on anesthesia was experimental in nature. What general skills of experimental design were necessary to plan effective experiments to test dosage measuring and delivery systems for anesthesia, or to investigate the properties and effects of different drugs?
In order for Snow to plan effective experiments to test dosage measuring and delivery systems for anesthesia, or to investigate the properties and effects of different drugs, he had to first decide that he would no longer be a surgeon, but focus his time primarily on his anesthesiology practices. The fact that Snow built a lab in his home, and used “animal subjects” for his testing, and recorded these findings, helped him investigate the properties and effects of different drugs. 5. Why are experiments considered strong tests of hypotheses? Experiments are considered strong tests of hypothesis because they are what scientists se to test a hypothesis. The use of experiments can find out if a hypothesis is true or not. Experiments can also lead to a change in a hypothesis. PART II QUESTIONS 1. What causes cholera, and how is it transmitted? Cholera is caused by the contamination of water by means of bacteria from partially digested food, feces, or decaying matter. It is transmitted through a contaminated water supply. 2. Why weren’t Snow’s ideas about cholera accepted at this early date? Snow’s ideas about cholera was not accepted well at this early date, because scientists were skeptical back then, and had to have some kind of definitive proof.
Also, they did not want to believe something that the scientific community did not condone or represent. 3. Explain why cholera outbreaks are more consistent with contamination of water than air. Snow had extensive experience with the use of gases, and none of which had any signs of relationship with the causes of cholera. Because of this fact, Snow knew it had to do something with the water supply. Cholera is a waterborne disease, it cannot be transmitted through the air. 4. Given that cholera outbreaks are more consistent with contamination of water than air, why did the miasma model persist?
The miasma model persisted because that is what the scientific community has agreed upon is the cause, and anything else was just a theory that was shunned and thought negatively upon. 5. How did Snow’s experimental research on anesthesia help him design a new model for the cause of cholera? When Snow had experimental research on anesthesia with animals, he found that the properties of gases were in no way consistent with the belief that cholera was caused by bacteria in the air. This led Snow to design a new hypothesis with water being the cause of cholera. . Why would evidence of cholera in people living side by side, differing only in water supply, provide critical evidence? The evidence of cholera in people living side by side, differing only in water, proves that cholera IS in fact transmitted by water, and not by the air. 7. When was the germ theory of disease proposed, and on what basis? The germ theory was proposed by Robert Koch and Louis Pasteur in the 1870s, on the basis that microorganisms were the cause of many diseases. PART III 1. Why was it useful to be able to verify the source of the water?
Verifying the source of water was the only way to prevent its spread. Since it is an epidemic, it affects many people at the same time, and it is very important to stop the spreading into more communities to prevent further death and the expansion of the disease. Verification allowed companies using that source to change where they obtained their water, and allowed water treatment to take place. 2. Why would a neighborhood served by two different water companies be more useful for testing Snow’s hypothesis than two neighborhoods each with their own source?
A reviewer in the London Medical Gazette suggested that Snow find people living side by side with lifestyles similar in all aspects except of their water source. He made this suggestion because this would ensure that the neighbors breathe the same air and resemble each other more in variables other than the water source. This would guarantee that the water is the source, and not other factors that is causing the cholera. 3. Epidemiologists often draw causal webs to illustrate the interrelationships among biological, social, and environmental variables that contribute to disease outbreaks.
Based on what you have learned so far, what variables should be included in a causal web for cholera? Based on what I have learned so far, the environmental variables that should be included in a casual web for cholera are areas which are near a body of water, which could overflow if contaminated and spread disease. Also, the text stated that cholera would be in remission due to the winter. This leads me to believe that temperature is a factor, since sunlight affects the persistence and spread of pathogens associated with phytoplankton and algae.
Humidity, because moisture allows pathogens to survive longer than if they were in a less humid or dry area such as a tropical climate. The social variables include if the area is urban or rural. Urban areas are more prone to diseases, since the chances of epidemics spreading, increases in more populated areas, which was evident in Killingworth Colliery and London. Also, areas where individuals immigrate often, which are typically in an urban setting, and can bring disease along with them.
In addition, if an area is upper or lower class, the lower class areas could be more prone to cholera because they tend to lack proper sanitation and sewage systems, allowing an overflow of wastes into the water. The biological variables that contribute to the cholera outbreak could include anything that is contaminated by excreted feces with cholera bacteria that is ingested. Food that is moist can be contaminated before or after preparation if left in room temperature for many hours, and provides the perfect breeding ground for the growth of bacteria.
Seafood from infected water that is insufficiently cooked, or fruits and vegetables grown at or near ground level which is washed or irrigated with contaminated water, and then eaten raw could definitely cause cholera. 4. Snow considered his conclusions about cholera to be inferences from observations whereas the reviewer from the medical journal considered these to be conjectures. What is the difference between inference and conjecture? An inference is the process of drawing a conclusion by applying clues of logic, and statistics to observations or hypotheses.
This represents Snow’s beliefs, since he used the knowledge that he gained from studying gases to determine that the symptoms that cholera patients had were inconsistent with the exposure to a toxic gas. The patients’ gastrointestinal symptoms led him to suspect that the cause of this disease was ingested rather than inhaled. This information, combined with the animalcules identified by early microscopists, allowed him to predict that cholera was indeed transmitted by water. A conjecture is a proposition that appears correct, but has not been proven or disproven.
This is indicative with the initial thought of cholera causing death by respiratory failure, which was thought to correspond with the miasma model. PART IV 1. The basic questions of epidemiology focus on time and place: why here and why now. What are the answers to these questions for the Broad Street outbreak? The answer to the Broad Street outbreak is the Broad Street pump. This pump was contaminated by a nearby seeping sewer system that allowed the cholera from infected baby diapers. Snow’s accomplice Whitehead received notice of the ill baby, which in turn prompted Snow to investigate.
This turned out to be the cause of the cholera outbreak, and gave Snow the definitive answer. 2. Epidemiology relies on non-experimental tests of hypotheses. What was Snow’s hypothesis and how did he test it? Snow’s hypothesis was, “The Broad Street Pump is the cause of the outbreak of cholera. ” Snow tested his hypothesis by going house to house knocking on doors in the surrounding area of the Broad Street pump. He found that most of the people that lived near this pump, and children that went to school on Broad Street, became sick with cholera. 3. How did Whitehead improve on Snow’s test of this hypothesis?
Whitehead inadvertently improved on Snow’s test of this hypothesis, by interviewing families in which no one had died from cholera. This showed that households using the Broad Street pump were nine times more likely to have cholera victims than those not using it. 4. What is the difference between correlation and causation? Correlation is defined as “a mutual relationship between two or more things”, meaning that they are related in some way. Causation would be “causing or producing a certain effect”, meaning it was just the cause with no relationship involved.
