Three Years After Exposure to a Virulent Form of the Flu

Three years after exposure to a virulent form of the flu, a population faces the same flu strain. The public health department determines that there are members of the population who have not been exposed to the flu strain. Which will the public health department predict will occur next?

A) The unexposed population will contract the illness
B) The unexposed population is not likely to contract the illness
C) The unexposed population will contract the illness and reinfect others
D) The flu will reach epidemic proportions and both populations will become ill