A 3 month old baby contracts a virus called whooping cough (pertussis) after receiving the first of a sequence of 3 necessary DPT vaccines designed to prevent 3 diseases (diphtheria, pertussis, and tetanus). The vaccines are given at 2, 4, and 6 months of age to allow the body to develop specific immunity against these diseases. After the infant recovered from the disease, the doctor informed the parents that from now on the infant would only need the DT (diphtheria, tetanus) part of the vaccine. Why does the infant only need the DT vaccine?

A. The infant had the first of a series of vaccines which caused the disease

B. The infant had the pertussis disease and has already produced antibodies against the disease.

C. The infant had the first of a series of vaccines which protected him from getting the disease.

D. The infant had the pertussis disease and will get diphtheria in the future.