The document discusses the epidemiological triad, which refers to the three factors required for a disease to occur: the agent, host, and environment. It provides details on each of these three factors. The agent can be a biological, nutritional, chemical, mechanical, or physical cause. The host refers to susceptibility factors like age, sex, and genetics. Environmental factors include physical, biological, and social aspects of a person's surroundings. Together, the presence of an agent, a susceptible host, and an environment conducive to transmission are necessary for a disease to occur.