California appears to be the hardest hit, but Midwestern states like Michigan and Ohio are also seeing more cases.
In California, pregnant women and people who come in contact with infants are being urged to get immunized against the highly contagious disease.
Whooping cough in adults can be treated with antibiotics, but it can be deadly among infants.
So far this year, there have seven infant deaths from whooping cough in California alone.
