The Wallingford-Swarthmore School District emailed parents on Friday to say it is taking precautions in the wake of a small outbreak of whooping cough.

Since the school year began, seven cases have been confirmed at Strath Haven High School, Strath Haven Middle School, and Swarthmore-Rutledge Elementary School. The students have been treated with antibiotics, the email from superintendent Lisa A. Palmer says.

Whooping cough, or pertussis, is a very contagious bacterial infection of the lungs that can be prevented by vaccination. The disease made a comeback beginning in the 1990s as the newer “acellular” vaccine was phased in. While safer than the older version, studies show its protective effect wanes, sometimes within a few years.

The Tdap vaccine, which protects against tetanus, diphtheria, and pertussis, is required for schoolchildren. Additionally, pregnant women and grandparents of infants are urged to get the vaccine because the main danger of pertussis is to babies. They can’t be vaccinated until they are 2 months old, and the coughing fits of the respiratory illness can be deadly for them. Grandparents have been a particular focus of public health campaigns on preventing whooping cough because most have not received the Tdap vaccine in many years.

The Wallingford-Swarthmore district has worked closely with the Pennsylvania Department of Health “to ensure all procedures related to pertussis are implemented,” Palmer said in the email. Enhanced cleaning has been done in the schools “in an abundance of caution,” even though infection from inanimate objects is not likely.

Pertussis cases tend to go in cycles, and many cases are not diagnosed or reported. The biggest U.S. outbreak in recent years was in 2012, when Pennsylvania had 1,945 cases and the nation had more than 48,000. The state reported 431 cases last year, while the country had 12,439.