2. Whooping Cough (Pertussis) What it is: A lung infection that makes it hard to breathe due to severe coughing. How you get it: People can breathe in the pertussis bacteria when someone who has whooping cough coughs or sneezes. Why it’s serious: It can be life-threatening, especially in babies less than 1 year old. Whooping cough can lead to pneumonia, seizures, and slowed or stopped breathing.