Cases of whooping cough are at their highest level in Wales since 2015.
Public health experts are appealing to pregnant women and parents of babies and young children to make sure they have been vaccinated against the infection.
The appeal comes after a "rapid increase" in cases over recent weeks.
Public Health Wales says there is a wave of increased infection every three to four years.
But after a reduction in cases during the pandemic, current levels have not been seen since 2015.
Cases confirmed in laboratories have not yet risen, but they are expected to increase as test results are reported.
Experts are concerned fewer pregnant women are being vaccinated against whooping cough, after the vaccination programme was introduced in the UK in 2013.
In the last year, vaccination rates have fallen from more than 80% to 70%.
The pertussis vaccine is given to babies at eight, 12 and 16 weeks old as part of the six-in-one vaccine.
They then get a booster at around three years and four months old.
Rates of the six-in-one vaccination remain high, Public Health Wales says.
Dr Christopher Johnson, from Public Health Wales, said whooping cough "can be very serious and lead to pneumonia and permanent brain damage".
"Young babies with whooping cough are at risk of dying from the disease," he added.
"We would urge all pregnant women and parents of babies and young children to ensure they take up their offer of vaccination when given, or to ask their GP, midwife or health visitor if they believe they may not have had it."