## **Core Concept**
Pertussis, also known as whooping cough, is a highly contagious respiratory tract infection caused by *Bordetella pertussis*. It primarily affects infants and young children, particularly those under the age of 5. The disease is characterized by a severe cough that can lead to complications, especially in this vulnerable age group.

## **Why the Correct Answer is Right**
The correct answer, **under 5 years**, is right because pertussis predominantly affects infants and young children. This age group is more susceptible due to incomplete vaccination or lack of immunity. The disease can lead to severe complications, such as pneumonia, encephalopathy, and death, particularly in infants under 6 months.

## **Why Each Wrong Option is Incorrect**
- **Option A:** *above 5 years* - This option is incorrect because, although pertussis can occur in older children and adults, it is less common and typically milder in these age groups due to waning immunity from childhood vaccination.
- **Option B:** *above 10 years* - This option is incorrect for similar reasons as option A; pertussis is less common in this age group, and when it does occur, it is often milder.
- **Option D:** *above 20 years* - This option is incorrect because adults can get pertussis, but it is not the primary age group affected; adults often serve as reservoirs for the bacteria, transmitting it to younger, more vulnerable individuals.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that pertussis is highly contagious and can have severe consequences in infants. Vaccination is crucial for prevention, and the typical vaccination schedule includes a series of DTaP (diphtheria, tetanus, and pertussis) vaccines starting at 2 months of age. Booster shots, such as the Tdap vaccine, are also recommended for adolescents and adults to maintain immunity and prevent transmission.

## **Correct Answer:** . under 5 years