Epidemiology is the study of health related states within a specified population. Since beef cattle are managed as a herd
or population, basic understanding of epidemiology is valuable tool to practitioners can utilize in beef practice. There is
a wide variation between animals within a herd in response to a disease. This variation leads to spectrum of clinical signs
from unaffected to sub-clinical to severe. Additionally, variation of risk factors such as virulence of infectious agents,
nutritional status and environmental conditions.
Epidemiological principals can be used to logically investigate a disease affecting a group of animals, to evaluate clinical
studies, and to interpret diagnostic tests. Applying epidemiological principals to clinical practice is usually referred to
as evidence-based medicine.
Figure 1
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Epidemiological Triad
Understanding and applying the epidemiological triad is fundamental in integrating epidemiology into clinical practice. Disease
results from complex interactions between the agent, the host and the environment (Figure 1). If the triad becomes imbalanced
then disease can result. Host factors include factors such as age, nutritional status, immune function, vaccine or exposure
status, pregnancy, stress etc. Environmental factors include temperature, climate, living conditions, density, ventilation,
animal movement, etc. Agent factors include virulence, dose, transmission, hardiness, etc. What is important to remember is
that husbandry and management can influence these factors (Figure 2).
Figure 2
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For example management strategy that provides clean dry calving environment can decrease disease incidence by improving the
health of the calf by allowing acquiring for passive transfer easier, keeping calf warm and dry and by preventing buildup
of pathogens. 
