What is the term for a disease-causing organism?

The term for a disease-causing organism is "pathogen." Pathogens are agents, such as bacteria, viruses, fungi, and parasites, that can lead to disease in their hosts. They are responsible for a variety of illnesses and infections, as they invade and replicate within the host's body, often leading to adverse health effects.

Understanding the nature of a pathogen is crucial in the study of biology and medicine, as it helps in the development of vaccines, treatments, and preventive measures for various diseases. By identifying and studying pathogens, scientists can better understand how they attack the host and devise strategies to combat them.

In contrast, the other terms provided refer to different concepts related to the immune system and disease. An "antigen" is a substance that can trigger an immune response but is not itself a disease-causing organism. An "allergen" is a specific type of antigen that causes an allergic reaction, and "infection" refers to the state resulting from the presence of pathogens in the body, not the pathogens themselves.