What is the term for an organism that is weakened and unable to cause disease?

The correct answer is "attenuated," which refers to an organism, particularly a virus or bacterium, that has been weakened or modified in such a way that it loses its ability to cause disease. This term is often used in the context of vaccines, where attenuated strains are introduced into the body to elicit an immune response without causing the actual disease. The process of attenuation can involve various methods, such as growing the organism in unfavorable conditions or through genetic modification.

While terms like "pathogenic" and "virulent" refer to organisms capable of causing disease, they do not imply any weakening or loss of harmful capability. The term "non-virulent" can describe organisms that are not harmful, but it does not specifically convey the idea of having been intentionally weakened as seen with attenuated strains used for immunization purposes. Therefore, "attenuated" is the most accurate term to describe an organism that has been deliberately rendered incapable of causing disease.