The bacterial infection also known as pertussis can be very serious for children under the age of 12 months. The biggest outbreak is currently in Washington State, where there were more than 3,000 cases through July 14. NBC's Robert Bazell reports.

Whooping cough is causing the worst epidemic seen in the United States in more than 50 years, health officials said Thursday, and they’re calling for mass vaccination of adults.

The epidemic has killed nine babies so far and babies are by far the most vulnerable to the disease, also known as pertussis, the U.S. Centers for Disease Control and Prevention says. The best way to protect them is to vaccinate the adults around them, and to vaccinate pregnant women so their babies are born with some immunity.

“As of today, nationwide nearly 18,000 cases have been reported to the CDC,” the CDC’s Dr. Anne Schuchat told reporters in a conference call. “That is nearly twice as many as reported last year. We may be on track for a record high pertussis rate this year,” she added.

“We may need to go back to 1959 to find as many cases. I think there may be more coming to a place near you.”

The last record year was 2010, when 27,000 cases were reported and 27 people died. In 1959, 40,000 cases were reported.

In 2008, whooping cough killed 195,000 people globally, according to the World Health Organization.

Whooping cough is caused by a bacterial infection. It gets its name from the nagging cough it causes that can make children breathless. They often gasp for air, making a distinctive whooping sound. But it’s not so serious in adults and they may not realize that a persistent cough is being caused by pertussis.

Related: Obesity may increase adults' whooping cough risk

Washington state is having an especially bad time with whooping cough this year, with 3,000 cases so far, compared to 20 at the same time last year, said Mary Selecky, secretary of the Washington State Department of Health. “For every case that we know about, we suspect that there are many people out there who have pertussis and don’t know it,’ Selecky said.

“In many cases, babies get this illness from their mothers or others close to them. It’s absolutely tragic.”

The state has distributed 27,000 doses of a booster vaccine for uninsured adults and has ordered more. “This disease is very easy to catch,” Selecky said. “It has certainly gotten hold of our population in Washington state.”

The CDC is trying to figure out what's going on, but Schuchat said a couple of factors are clearly at work. The formulation for the whooping cough vaccine was changed in 1997, and kids hitting age 13 and 14 now are the first to have been fully vaccinated with five doses of the new vaccine. The new formulation causes less of a reaction, but it may also wear off sooner, Schuchat said.

The older vaccine was made using a whole pertussis bacterium. It was very effective, but it did cause swelling in some kids who got it, and sometimes caused a fever -- something that scared parents. It also was widely blamed for causing rare but serious neurological reactions, although Schuchat said studies have not confirmed this.

“Vaccines have done a good job of reducing the incidence of pertussis but our vaccines aren’t perfect,” Schuchat said. “We wish we had better ways of controlling pertussis. Given how dangerous pertussis is for babies, preventing infection in babies is our priority.”

Schuchat says people who are not vaccinated have eight times the risk of infection compared to people who are fully vaccinated against whooping cough. And if someone who’s been vaccinated does get whooping cough, the disease is usually less serious and they are far less likely to infect someone else.

The CDC says 95 percent of toddlers aged up to three years have received at least three doses of the vaccine and 84 percent have four doses. And in 2010 69 percent of 13- to 17-year-olds got a fifth booster dose. Kids should get five doses to be fully protected.

And while adults are supposed to have at least one dose of whooping cough vaccine, only 8.2 percent of U.S. adults have done so.

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