**Core Concept**
The secondary attack rate refers to the proportion of exposed contacts who develop the disease within a specified period after exposure to a primary case. In the context of pertussis (whooping cough), the secondary attack rate in unimmunized household contacts is particularly high due to the highly contagious nature of the disease.

**Why the Correct Answer is Right**
Pertussis is caused by the bacterium *Bordetella pertussis*, which spreads primarily through respiratory droplets. Unimmunized household contacts are at a higher risk of contracting the disease, as they have not developed immunity through vaccination or previous infection. The secondary attack rate in this population is estimated to be around 90%, indicating a high likelihood of disease transmission. This is because unimmunized individuals lack the protective antibodies against pertussis, making them more susceptible to infection.

**Why Each Wrong Option is Incorrect**
* **Option A:** 30% is too low, considering the high contagiousness of pertussis and the lack of immunity in unimmunized contacts.
* **Option B:** 40% is still underestimating the actual secondary attack rate, which is significantly higher in this population.
* **Option C:** 60% is closer to the actual value but still underestimates the risk of disease transmission among unimmunized household contacts.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the secondary attack rate of pertussis can be significantly reduced through vaccination, especially in high-risk groups such as infants and young children. Healthcare providers should emphasize the importance of immunization and provide guidance on post-exposure prophylaxis to prevent disease transmission.

**✓ Correct Answer: D. 90%**