What term describes an organism that can cause a disease?

The term that describes an organism capable of causing disease is "pathogen." Pathogens can include a variety of microorganisms such as bacteria, viruses, fungi, and parasites. They have the ability to invade host organisms and disrupt normal physiological functions, leading to disease. Understanding pathogens is essential in the field of wastewater treatment, as they can affect public health and the safety of water supplies.

The other terms listed do not fully encapsulate this definition. "Contagion" refers specifically to the transmission of a disease from one person to another and does not denote the organism itself. "Infection" describes the process through which a pathogen enters and multiplies in a host but also does not refer to the organism. "Virus" is a type of pathogen, but it is not a comprehensive term, as not all pathogens are viruses. Hence, "pathogen" is the most accurate term to describe an organism that can cause disease.