In Terms of Community Ecology, Why Are Pathogens Often More

In terms of community ecology, why are pathogens often more virulent now than before?

A) More new pathogens have recently evolved.
B) Host organisms have become more susceptible because of weakened immune systems.
C) Human activities are transporting pathogens into new habitats (or communities) at an unprecedented rate.
D) Medicines for treating pathogenic disease are in short supply.
E) Sequencing of genes in pathogenic organisms is particularly difficult.