Studies of virulence by the human pathogen Bordetella pertussis, the causative agent of whooping cough, has been shown to be controlled by the bvgAS operon. Induction of virulence-related genes only occurs when B. pertussis is grown at 37°C and certain bvgS mutations result in a loss of the temperature response. DNA sequence analysis indicated that the bvgS gene product contains a potential membrane spanning region. In addition, all bvgA mutants incapable of causing virulence had an aspartate residue changed to alanine. Based on this information propose a model for how bvgA and bvgS play a role in B. pertussis virulence.

parasites can detrimentally affect their host, but also must not allow their host to die before they can complete their life cycle. some parasites have complex life cycles with many different hosts, and have had to find ways of persisting in many different environments during the change from one host to another. ectoparasites live outside the body of the host, and can more easily  disperse, but are  more exposed to predators, parasites, parasitoids. endoparasites live inside the body of the host, and are therefore protected from the outside environment. but they can be  attacked by the host's immune system. they have therefore developed  mechanisms for dispersal such as  complex life cycles.

well, that's easy; the answer's c, "it reduces contamination of the water and substrate of the estuary."