Since the beginning of the year, 5,978 confirmed, probable and suspected cases of the disease have been reported in California.

All of the deaths occurred in infants under the age of 3 months, says Michael Sicilia, a spokesman for the California Department of Public Health. Nine were younger than 8 weeks old, which means they were too young to have been vaccinated against this highly contagious bacterial disease.

"This is a preventable disease," says Sicilia, because there is a vaccine for whooping cough to protect those coming in contact with infants, and thereby protect the infants.

However, some parents are choosing to not vaccinate their children. In other cases, previously vaccinated children and adults may have lost their immunity because the vaccine has worn off.