Fact 1: Harming an animal species has a negative impact on the population size of that species.  Fact 2: Some species of amoeba are harmful parasites.  Given the two facts above, what can negatively impact a species?

OPT: [I] chemicals. [II] elephant. [III] cells. [IV] dog. [V] drought. [VI] mines. [VII] steroids. [VIII] amoeba.
Answer:
[VIII]