A WHOOPING cough epidemic has led NSW to record its highest number of new cases of the disease.

More than 13,000 people were infected in 2011, and one baby died, surveillance figures published in the CSIRO's NSW Public Health Bulletin show.

Experts do not know how to stop the spread of the disease, as even people who are vaccinated are at risk because the vaccine begins to wear off after only a couple of years.

Alexander Rosewell, who led the surveillance report team, said 2011 was an epidemic year.

"It is largely due to a build-up of susceptible people in the community," he said. "When there are low levels of protective antibodies in the community you are more likely to see an outbreak."