According to the passage, virgin-soil epidemics can be distinguished from other catastrophic out- breaks of diseases in that virgin-soil epidemics______.
[A] recur more frequently than chronic diseases
[C] usually involve a number of interacting diseases
[B] involve populations with no prior exposure to a disease
[D] are less responsive to medical treatment than are other diseases

A．According to the passage, virgin-soil epidemics can be distinguished from other catastrophic out- breaks of diseases in that virgin-soil epidemics______.
B．recur more frequently than chronic diseases
C．involve populations with no prior exposure to a disease
D．usually involve a number of interacting diseases
E．are less responsive to medical treatment than are other diseases