Which of the following describes a role of vectors in disease transmission?

Vectors play a crucial role in the transmission of diseases by serving as hosts for pathogens, which is why this option is correct. In the context of infectious diseases, vectors are typically living organisms, such as mosquitoes, ticks, or fleas, that carry and transmit pathogens (such as viruses, bacteria, and parasites) to humans or other hosts. The vector itself does not usually succumb to the infection but provides an essential means for the pathogen to move from one host to another, thus facilitating the spread of the disease.

Other options relate to different mechanisms of disease prevention or transmission but do not accurately describe the role of vectors. For example, while immunity and barriers can protect against infections, they do not involve the involvement of vectors in the spread of disease. Facilitating fluid exchange generally pertains to physiological functions rather than the specific mode of disease transmission that vectors represent.