It's a sound that many thought was long gone, but it's a sound that has been creeping back up in the United States.
Colorado has reported more than 1,000 cases of it this year alone.
It's the sound of whooping cough.
According to Dr. Jeffrey Thomas from Lovelace Health Center, whooping cough is caused by the bacterium Bortetella pertussis.
"It's a highly contagious disease that's spread from person to person through respiratory droplets, like sneezing and coughing," Thomas said.