Which of the following best describes mechanical transmission of a disease?

The best description of mechanical transmission of a disease is that it involves passive carrying without multiplication of the organism. In this type of transmission, a vector, such as an insect, carries a pathogen from one host to another without any change or development of the pathogen within the vector. Instead, the microorganism remains externally on the vector's body or mouthparts and is transferred directly to a new host, typically when the vector comes into contact with that host.

This distinction is critical in understanding how diseases spread. For instance, flies can mechanically transmit pathogens by landing on contaminated materials and then moving to food. In these cases, the pathogen does not replicate within the fly but rather is merely transferred from one surface to another.

The other options describe different concepts regarding disease transmission that do not accurately capture the nature of mechanical transmission. For example, active participation by the host, transformation of the organism, or involvement of immune responses are more representative of other transmission dynamics such as biological transmission or host-pathogen interactions.