Twenty-one students across both campuses now have Pertussis, or whooping cough, according to an email that Community Health Officer Milo Sini sent Wednesday to parents of students in classes with those diagnosed.

Whooping cough is a highly contagious respiratory disease that spreads through coughing and sneezing, according to County of Los Angeles Public Health.

Symptoms sometimes do not manifest for up to three weeks, Sini said in the email, and can appear to belong to a common cold.

Since symptoms are often minimal in the first three weeks, especially for people who have received a vaccination, some students come to school while they are a carrier for Pertussis without knowing it, causing it to spread more quickly. Vaccinations for whooping cough wear out every 10 years, and, according to the email, are only effective for eight or nine out of 10 people who receive it.

“In order to safeguard our community and minimize the number of people affected, we want to remind you, that whenever your child has a medical issue, especially one that is known to be contagious, it is essential you inform the school immediately,” Sini said in the email.

In addition to informing the school, families must provide the school with a doctor’s note, Sini said in the email.