Because of their young age, immature immune system and delicate lungs, they are also more likely to have problems such as pneumonia, apnea (pauses or stoppages in breathing) or seizures related to the infection.

CNN: Can breast-feeding prevent whooping cough?

Their lungs and brain and skin are immature, they are very vulnerable to infection and we can't get (intravenous) lines in to deliver the medicine they need.

BBC: Health