Which of the Following Best Defines an Endemic Disease

Which of the following best defines an endemic disease?

A) It is a disease that has a much higher incidence than usual.
B) It is a disease that used to be common but is eradicated through vaccination programs.
C) It is a disease that is transmitted by an arthropod vector.
D) It is a disease that occurs naturally in animals but may spread to people.
E) It is a disease that is always present at some level in a population.