Components of the Triad

When considering the Epidemiological Triad, there are three main factors that are taken into account: The Agent, The Host, and The Environment. Below is a brief description of each factor.


The agent within the epidemiological triad is microbes that cause disease to occur. When considering infectious diseases, the agent is an external microorganism that needs to be present in order for the disease to occur. These pathogens include viruses, bacteria, fungi, parasites, and other microorganisms. Many people refer the agent as the “germ.” Over time, the concept of the agent has expanded to allow for application of the triad to non-infectious diseases and accidents. To accommodate for this expansion, the agent also includes both chemical and physical disease-causing factors like asbestos and cigarette smoking (CDC, 2012).


The host within the triad refers to the human capable of developing the disease. Numerous risk factors contribute to a human’s exposure, susceptibility, and response to a harmful agent. Exposure to an agent is influenced by a person’s behavior like sexual practices, hygiene, and other behavioral practices. Exposure is also influenced by biological/genetic factors such as age and gender. Susceptibility and response are both influenced by factors like genetic composition, physiological make up, and health behaviors such as nutrition and exercise (CDC, 2012).


The environment in the part of the epidemiological triad that addresses everything that is external to the host. Environmental factors affecting the contraction of a disease include factors like geographic location and climate (CDC, 2012). Environmental factors that contribute to the transmission of an agent include, but are not limited to, biological factors and socioeconomic factors. Biological factors refer to the animals present in the area that transmit the agent, and socioeconomic factors refer to issues like crowding, poor sanitation, and accessibility to health care (CDC, 2012). The environment also includes social norms that affect the spread of disease (i.e. condom use). All aspects of the environment around the host are key factors to consider when trying to determine the cause of a disease.