**Core Concept**
Newborns acquire immunity against certain diseases transplacentally from their mothers, providing temporary protection against infections in the first few months of life. This transplacentally acquired immunity is primarily due to the presence of antibodies in the mother's blood, which are transferred to the fetus through the placenta.

**Why the Correct Answer is Right**
Pertussis, also known as whooping cough, is a highly contagious bacterial infection caused by Bordetella pertussis. The immunity against pertussis is not transferred transplacentally from the mother, unlike other diseases such as measles, diphtheria, and poliomyelitis. This is because the immunity against pertussis is primarily cell-mediated, involving the action of T lymphocytes, which are not transferred from the mother to the fetus. Therefore, newborns are not protected against pertussis through transplacentally acquired immunity.

**Why Each Wrong Option is Incorrect**
**Option A:** Measles - Measles immunity is transferred transplacentally, providing protection to newborns against measles in the first few months of life.
**Option C:** Diphtheria - Diphtheria immunity is also transferred transplacentally, offering protection to newborns against diphtheria in the first few months of life.
**Option D:** Poliomyelitis - Poliovirus antibodies are transferred from the mother to the fetus, providing temporary protection against poliomyelitis in the first few months of life.

**Clinical Pearl / High-Yield Fact**
It's essential to note that while transplacentally acquired immunity provides temporary protection against certain diseases, it is not a substitute for vaccination. Newborns should receive their first vaccinations at birth and continue to receive subsequent vaccinations according to the recommended schedule to ensure long-term protection against infectious diseases.

✓ Correct Answer: B. Pertussis