In pneumonic plague, transmission occurs from human to human by
**Core Concept**
Pneumonic plague is a severe form of plague, caused by the bacterium *Yersinia pestis*, which can be transmitted between humans. The primary mechanism of transmission in pneumonic plague involves respiratory droplets.
**Why the Correct Answer is Right**
The correct answer involves respiratory droplets, as pneumonic plague can be spread from person to person through the air when an infected person coughs. This is because *Yersinia pestis* bacteria can be found in the droplets expelled during coughing, allowing for human-to-human transmission without the need for a vector like fleas.
**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it does not involve direct human-to-human transmission.
**Option B:** Incorrect as it is not the primary method of transmission for pneumonic plague.
**Option C:** Incorrect because, although vectors can transmit plague, they are not primarily involved in human-to-human transmission of the pneumonic form.
**Clinical Pearl / High-Yield Fact**
A key point to remember is that pneumonic plague can be highly infectious and deadly if not treated promptly, making rapid diagnosis and treatment crucial.
**Correct Answer:** D. respiratory droplets.