Certified Crop Advisor Practice Exam 2025 – Complete Study Resource

The disease triangle is a model used to understand the interactions between three key components necessary for disease to occur: the host, the pathogen, and the environment.

The host is the organism that can be infected by the pathogen, providing the necessary biological environment for the pathogen to thrive and cause disease. The pathogen is the harmful agent – be it a virus, bacterium, fungus, or nematode – that leads to disease in the host. The environment encompasses the external conditions that affect the host and pathogen, such as weather, soil conditions, and agricultural practices, which can influence the disease's development and severity.

In this context, an antigen is a substance that the immune system recognizes as foreign and can provoke an immune response. While antigens are essential in the study of immunology and can influence how hosts respond to pathogens, they are not a component of the disease triangle. Hence, the inclusion of the pathogen, host, and environment leads to a comprehensive understanding of the conditions necessary for the onset of disease, while antigens do not fit into this framework, making it the correct answer.