**Core Concept**
Pertussis, also known as whooping cough, is a highly contagious respiratory tract infection caused by the bacterium *Bordetella pertussis*. The disease primarily affects infants and young children, causing severe coughing fits that can lead to complications such as pneumonia and brain damage.

**Why the Correct Answer is Right**
Pertussis is most severe in infants under 6 months of age due to their immature immune systems and incomplete vaccination status. Infants are not fully protected by maternal antibodies, and their own immune response takes time to develop. As a result, they are more susceptible to severe illness and complications. The correct answer is a specific age group that reflects this vulnerability.

**Why Each Wrong Option is Incorrect**
**Option A:**
This option is too broad, as pertussis can affect individuals across various age groups, including adolescents and adults.

**Option B:**
While pertussis can also affect older children and adolescents, it is not the most vulnerable age group.

**Option C:**
This option is partially correct, as pertussis can indeed affect young children. However, it does not specify the most vulnerable age group within this category.

**Clinical Pearl / High-Yield Fact**
A key aspect of pertussis management is vaccination, particularly in infants. The DTaP vaccine is recommended for children starting at 2 months of age, with a series of doses administered at 2, 4, 6, and 15-18 months, and a booster dose at 4-6 years of age.

**Correct Answer:** C. Young children, especially those under 6 months of age.