Percent positiveInfluenza positive tests reported to CDC by U.S. clinical laboratories,
national summary, 2016–2017 seasonWeek12,00010,0008,0006,0004,0002,000201640201642201644201646201648201650201652201702201704201706201708201710201712201714201716201718201720201722201724201726201728201730201732201734201736201738026(^2524)
(^2322)
(^2120)
(^1918)
17
(^1615)
(^1413)
(^1211)
(^109)
(^87)
6
(^54)
(^32)
(^10)
Number of positive specimens
A
B
Percent positive
% Positive Flu A
% Positive Flu B
FIGURE 8-3
Influenza Positive Tests Reported to CDC by U.S. WHO/
NREVSS Collaborating Laboratories, National Summary,
2016–2017
Finally, the frequency of diseases may also be described by short-term trends,
which are brief, unexpected changes in disease distribution. Short-term changes
include epidemics and pandemics. To understand epidemic and pandemic, it
is important first to understand the definition of endemic. Endemic describes
the expected occurrence of a particular disease within a community or popu-
lation. Epidemic is a widespread occurrence of a disease in a community or
population that is in excess of what is expected. Pandemic is an epidemic that
has spread worldwide. For example, in 2012 and 2013, Japan experienced an
epidemic of rubella.
Until the early 2000s, rubella was endemic in Japan, with periodic epidem-
ics approximately every 5 years and seasonal increases in the spring and
summer. The number of reported rubella cases remained at record low
levels until 2010 [n = 87], and in 2011 [n = 378], a few outbreaks were
KEY TERMS
endemic:
The expected
occurrence of a
particular disease
within a community
or population
epidemic: A
widespread
occurrence of
a disease in a
community or
population that is
in excess of what is
expected
Reproduced from Centers for Disease Control and Prevention. (2017). Weekly U.S. influenza
surveillance report. 2016-2017 Influenza season week 31 ending August 5, 2017. Retrieved from
https://www.cdc.gov/flu/weekly/index.htm#whomap
8.4 Descriptive Epidemiology 203