A 29-year-old previously healthy man was referred to the University
of California at Los Angeles (UCLA) Medical Center with a history
of fever, fatigue, lymph node enlargement, and weight loss of almost
25 lb over the preceding 8 months. He had a temperature of 39.5°C,
appeared physically wasted, and had swollen lymph nodes. Laboratory
evaluation revealed a depressed level of peripheral blood lymphocytes.
The patient suffered from simultaneous infections involving Candida
albicans in his upper digestive tract, cytomegalovirus in his urinary
tract, and Pneumocystis carinii in his lungs. Although antibiotic
therapy was administered, the patient remained severely ill.
Epidemiology is a fundamental medical science that focuses on
the distribution and determinants of disease frequency in human
populations. Specifically, epidemiologists examine patterns of illness
in the population and then try to determine why certain groups or
individuals develop a particular disease whereas others do not.
Knowledge about who is likely to develop a particular disease
and under what circumstances they are likely to develop it is central
to the daily practice of medicine and to efforts to improve the health
of the public. To prevent an illness, health care providers must
be able both to identify persons who because of personal characteristics
or their environment are at high risk, and to intervene to reduce
that risk. This type of knowledge emerges in many cases from epidemiologic research.
This book serves as an introduction to epidemiologic methods
and the ways in which these methods can be used to answer key medical
and public health questions. This chapter begins by considering
a particular disease, as described in the Patient Profile. Focusing
attention on one disease enables us to demonstrate the important
contribution of epidemiology to current knowledge about this condition.
Although the emphasis is on a single disease, it should be recognized
that epidemiologic methods can be applied to a wide spectrum of
conditions, ranging from acute illnesses, such as outbreaks of food-borne
infections, to long-term debilitating conditions, such as Alzheimer’s
The man in the Patient Profile was referred to the UCLA Medical
Center in June 1981. At the time, there was no obvious explanation
as to why a healthy young man would suddenly develop concurrent