## Core Concept
Whooping cough, also known as **pertussis**, is a highly contagious respiratory tract infection caused by the bacterium *Bordetella pertussis*. The primary mode of transmission is through respiratory droplets spread when an infected person coughs or sneezes. The infection is most commonly seen in children but can also affect adults.

## Why the Correct Answer is Right
The correct answer, **. (an infected child)**, is right because whooping cough is most commonly seen in children, and an infected child is the most frequent source of infection for others, including family members, caregivers, and community contacts. *Bordetella pertussis* bacteria colonize the respiratory tract, leading to the characteristic symptoms of whooping cough, which include severe coughing fits followed by a distinctive "whoop" sound during inhalation.

## Why Each Wrong Option is Incorrect
- **Option A:** An infected adult can indeed spread whooping cough, but they are less likely to be the source of infection compared to an infected child, especially in settings where vaccination rates are high and adults may have waned immunity.
- **Option B:** While *Bordetella parapertussis* can cause a similar illness known as parapertussis, it is less common than *B. pertussis* and typically causes a milder form of the disease. It is not the most common source of infection for whooping cough.
- **Option D:** The bacteria themselves are not a source of infection in the context meant by the question; rather, an infected individual (usually a child) is the source.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **the incubation period of whooping cough is usually 7-10 days but can range from 5-21 days**. During this time, the infected individual can spread the disease to others before the onset of symptoms. Vaccination is crucial for prevention, and booster doses are recommended for adolescents and adults to maintain immunity.

## Correct Answer: B. an infected child