EPIDEMIOLOGY 369
of the disease, if all 15 had the fl u and someone contracted
the disease twice, incidence would be 16 times in a popula-
tion of 15, with prevalence being 15 out of 15. As noted,
very seldom will the incidence be larger than prevalence;
this would only occur in rare or unusual events and would
likely involve small populations. It is important to under-
stand the difference between each of these terms, in that
they represent different “values” for a disease or event in the
population being studied. Table 1 provides an example of
incidence and prevalence for data collected from a computer
database of different diseases (Centers for Disease Control
and Prevention [CDC], 2004).
In Table 1, the incidence and prevalence (I/P) are
the same, since both involve the occurrence of death. For
Parkinson’s disease there was an increase in I/P for both the
United States and Pennsylvania, while for cancer there was
a decrease (1970–2000) for the United States and a steady
state for Pennsylvania. Adjustment involves standardizing
the population for such variables as age, race, and sex. These
variables can also be considered confounders.
Epidemiology was recognized at fi rst implicitly by a
general appreciation of probabilities, rather than explicitly
by recording each incident. This is noted by some of the fi rst
attempts to conduct epidemiological investigations where
the number of events was noted but no rate of the event was
determined. Just knowing the number of cases alone, with-
out a rate of occurrence, does not allow comparison with
other events. However, lack of a rate does not necessary
minimize an epidemiological study, although in the modern
day, rates are often essential. But, in parallel cases with the
base population among whom the cases have occurred, in
order to obtain in ratio from the rate incidence or occurrence
of the disease, suitably refi ned, according to the circumstancesof the situation and in ways we shall discuss later, such a
rate can be used as a measure for purposes of comparison in
the same place between different time periods, or between
different places at the same time, or in a variety of other
ways. Rates are represented in units of a population, like
per 100,000 people. By an appropriate extension we can
measure the impact of disease, whether in general or of a
particular type, on the population. But we also fi nd that the
characteristics of the population itself can alter the manifes-
tation of the disease, so that the science of epidemiology can
be symmetrically defi ned as measuring the impact of disease
on a population, or of a population on a disease—perhaps
better expressed by saying that the concern of epidemiology
is with the measurement of the interaction of disease and
population. Thus, at the heart of epidemiology is counting
(Lange et al., 2003a), which is then concerted to a rate as
expressed as either incidence of prevalence.
The issues of rates can be illustrated through two his-
torical studies. The fi rst did not employ rates in determining
a cause of scurvy, while the other employed rates to locate
the source of the infectious agent in causing cholera. These
studies illustrate how rates can be used in evaluating disease,
although the importance of basic observation cannot be for-
gotten or lost in a study.
In the study by James Lind on scurvy (Timmreck,
1998), in 1753, he noted that some sailors developed this
disease while others did not. Lind examined the diet of
those with and without the disease as part of the investiga-
tion into the cause of scurvy. Although he did identify a
crude rate in a population of sailors initially studied (80 out
of 350 had the disease), this rate or its comparison was not
employed in his study design. To evaluate the differences in
reported diets, he provided oranges and lemons to two sail-
ors and followed their progress. After a few days he noted
that their scurvy subsided and concluded that these dietary
supplements were most effective at treating and preventing
the disease. In modern epidemiology we would most likely
look at the rates of disease occurrence and cure rather than
using observational numbers, as had been used by Dr. Lind.
However, Dr. Lind did make observations of cause and
effect and time and place, as well as sources of causation
in the disease process (Timmreck, 1998). It is worth noting
that today the size of this study would likely be considered
too small for publication in a scientifi c journal. However,
this demonstrates the importance of observation even for
small study populations.
What most consider the fi rst true epidemiology study that
employed rates was conducted by John Snow in the 1850s
and concerned an outbreak of cholera. Dr. Snow actually
conducted two studies on the epidemiology of cholera: the
fi rst was a descriptive study in the SoHo district of London
(this is in the Broad Street area), and the second was a clas-
sical investigation in determining rates of disease.
In the fi rst study he observed that two different popula-
tions were affected by cholera, one with a low number of
deaths and the other with a high number. By mapping loca-
tions of deaths, commonly used today in geographic and eco-
logical epidemiology studies, he concluded that there wereTABLE 1
All races and all gender death rates for Parkinson’s disease and cancer
of bronchus and lung unspecified for 1979–1998 using 1970 and 2000
standardized populationsParkinson’s DiseaseStandard Population Region Crude* Age-adjusted*
2000 US 4.8 4.9
1970 US 2.9 2.2
2000 Pennsylvania 3.5 3.3
1970 Pennsylvania 3.5 2.2
Cancer of Bronchus and Lung UnspecifiedStandard Population Region Crude* Age-adjusted*
2000 US 52.5 55.2
1970 US 46.0 60.7
2000 Pennsylvania 54.4 60.7
1970 Pennsylvania 46.0 60.7
Source: From CDC (2004), CDC Wonder (database on disease occurrence).
* Rates are per 100,000.C005_011_r03.indd 369C005_011_r03.indd 369 11/18/2005 10:25:39 AM11/18/2005 10:25:39 AM