What is the term for an organism that causes disease?

The term for an organism that causes disease is "pathogen." Pathogens are microorganisms such as bacteria, viruses, fungi, and parasites that can invade a host organism and disrupt normal bodily functions, resulting in disease. The ability of a pathogen to cause disease depends on various factors, including its virulence, the mode of transmission, and the host’s immune response.

While the other choices relate to the context of disease transmission and infection, they represent different roles in the disease process. A host is the organism that is infected by the pathogen, a carrier is an individual who harbors the pathogen without exhibiting symptoms of the disease but can still transmit it to others, and a vector is an organism that transmits a pathogen from one host to another, often without being affected by the disease itself. Understanding the specific role of pathogens is crucial in fields like veterinary science, where mitigating the impact of infectious diseases is essential for animal health and public safety.