A Wild Mouse Population Has Coexisted for Millions of Years

A wild mouse population has coexisted for millions of years with Toxoplasma parasites.However, during a recent year, the mouse population was almost decimated by a Toxoplasma infection.What factor most likely explains this sudden change in the pattern of coexistence?

A) A new, more virulent strain of Toxoplasma emerged.
B) The existing Toxoplasma strain became more virulent.
C) The existing Toxoplasma strain began to reproduce more rapidly.
D) The mouse population became more susceptible to the original Toxoplasma parasite.
E) The mouse population became less susceptible to the original Toxoplasma parasite.