Microbiology, 2021

16.2 • Tracking Infectious Diseases 661
Figure 16.8
Joseph Lister initiated the use of a carbolic acid (phenol) during surgeries. This illustration of a surgery shows a pressurized
canister of carbolic acid being sprayed over the surgical site.
LINK TO LEARNING
John Snow’s own account of his work (https://openstax.org/l/22JohnSnowacco) has additional links and
information.
This CDC resource (https://openstax.org/l/22CDCpointsourc) further breaks down the pattern expected from a
point-source outbreak.
Learn more about Nightingale’s wedge chart (https://openstax.org/l/22nightwedgecha) here.
CHECK YOUR UNDERSTANDING
• Explain the difference between common source spread and propagated spread of disease.
• Describe how the observations of John Snow, Florence Nightingale, and Joseph Lister led to improvements
in public health.
Types of Epidemiological Studies
Today, epidemiologists make use of study designs, the manner in which data are gathered to test a hypothesis,
similar to those of researchers studying other phenomena that occur in populations. These approaches can be
divided into observational studies (in which subjects are not manipulated) and experimental studies (in which
subjects are manipulated). Collectively, these studies give modern-day epidemiologists multiple tools for
exploring the connections between infectious diseases and the populations of susceptible individuals they
might infect.
Observational Studies
In an observational study, data are gathered from study participants through measurements (such as
physiological variables like white blood cell count), or answers to questions in interviews (such as recent travel
or exercise frequency). The subjects in an observational study are typically chosen at random from a
population of affected or unaffected individuals. However, the subjects in an observational study are in no way
manipulated by the researcher. Observational studies are typically easier to carry out than experimental
studies, and in certain situations they may be the only studies possible for ethical reasons.
Observational studies are only able to measure associations between disease occurrence and possible
causative agents; they do not necessarily prove a causal relationship. For example, suppose a study finds an
association between heavy coffee drinking and lower incidence of skin cancer. This might suggest that coffee
prevents skin cancer, but there may be another unmeasured factor involved, such as the amount of sun
exposure the participants receive. If it turns out that coffee drinkers work more in offices and spend less time
outside in the sun than those who drink less coffee, then it may be possible that the lower rate of skin cancer is