As cases of pertussis, or as it's more commonly known, "whooping cough" spread in the western U.S., the CDC is issuing a warning that adults who have not had a pertussis booster shot after the age of 18 should get one, or risk catching the disease and helping it spread to infants and children. CBS News reports that hundreds of cases have been reported in nine states, and that Washington state has declared an epidemic.

The disease killed thousands of people every year, mostly children, until an effective vaccine was created in 1940. There have been periodic outbreaks of the disease in the U.S., but a combination of factors, including the high number of parents in Washington who have chosen not to vaccinate their children, have decreased what epidemiologists call "herd immunity," and made the population vulnerable to this outbreak.

Herd immunity is what happens when a high enough portion of the population is vaccinated that even individuals who are unvaccinated are conferred the benefits of immunity. Because so many members of the community are immune, a disease's routes of transmission are broken. In Washington, herd immunity has broken down under the combined stresses of a relatively high number of un-immunized children, a health care infrastructure worn down by years of budget cuts and a higher-than-average percentage of families with no health insurance to create a mushrooming public health crisis.

The New York Times reported that Washington passed a law only last year requiring parents who decline vaccinations for their children produce written proof that they had consulted a physician first.  Washington State's secretary of health Mary Selecky told the Times, "We had the easiest opt-out law in the nation until last year, so what we also had was the highest percent of parents opting out."

And now, the highest number of pertussis cases in the nation.  According to the CDC, "In Washington, there have been 1,284 cases reported statewide through May 5, 2012, compared to 128 reported cases in 2011 during the same time period. There were 965 cases reported statewide in 2011 compared to 608 reported cases in 2010."

On the whole, the bulk of children in the U.S. are protected against pertussis by early childhood vaccinations, but vaccines wear off over time (.pdf). Adults should get a booster shot after age 18, but according to CBS, only 8 percent of adults are current on their pertussis vaccinations.

The disease is caused by the bacteria Bordetella pertussis and begins as a regular cold with sneezing, runny nose, congestion and maybe a mild cough or fever, but after a week or two, the coughing fits begin, fits so deep and severe that all air is forced from the lungs, forcing the patient to gaspingly inhale, hence the "whooping" sound that gives the disease its name. In infants the symptoms can be more subtle, but even more dangerous. Some infants don't cough at all, but rather have episodes of apnea, a break in the child's regular breathing pattern. Parents are advised that if their infant child ever appears to have difficulty breathing, take the baby to a doctor or hospital right away.

The CDC says that more than half of babies under 1 year of age who contract pertussis must be hospitalized. About 1 in 5 babies with pertussis will develop pneumonia and 1 in 100 will have seizures as a result of the disease. In approximately 1 out of 100 cases, the baby will die.

The organization recommends vaccinations for infants and children, and booster shots for pregnant women and any adults who are going to have contact with infants and children.

Watch this video about the outbreak, which originally aired on CBS on May 15, 2012, below: