A Bacterial Disease Previously Known to Only Infect Dogs Begins

A bacterial disease previously known to only infect dogs begins to cause respiratory infections in humans who are in close contact with infected dogs.Which of the following explanations most likely explains the emergence of this new human pathogen?

A) The genome of the bacterium changed through horizontal gene transfer within the microbiota of the dog.
B) There was a mutation in the human genome that allowed the bacterium to attack human cells more vigorously.
C) The bacterium became infected by a mutation-inducing virus transmitted through flea bites on the dog.
D) The bacterium forms dormant endospores that remain in the environment even after the infected dogs are removed.