Nine cases of measles have so far been recorded in an outbreak in California and Utah, and the number is expected to rise. It is thought that many of the infections were transmitted at Disneyland. Predictably, most of those who have become sick were not vaccinated. Infuriatingly, two of them were children who were too young to get the measles shots, rather than being the children of vaccine opponents.

In addition to the nine confirmed cases, health officials say that three more are suspected. “If you have symptoms, and believe you may have been exposed, please contact your healthcare provider,” California state health officer Ron Chapman said. He added, “The best way to prevent measles and its spread is to get vaccinated.”

Unlike Ebola, measles is highly contagious. When its airborne, a single person can potentially infect many others by coughing or sneezing, particularly if they spend a day in a crowded place like Disneyland. If you spend long enough with someone who has measles and you either have not had it before or been vaccinated, you have a 90% chance of coming down with the disease.

Fortunately, most of the American population have been vaccinated. Although no vaccine is 100% effective, very few of the vaccinated people who attended Disneyland on the relevant days are likely to get sick, even if they spent hours in a queue in front of someone who was contagious.

On the other hand, the Centers for Disease Control and Prevention (CDC) recommend vaccination at 12-18 months of age. Until that time, children are highly susceptible. Two of the those infected during the Disneyland outbreak were under this age.

While measles is frequently fatal where healthcare facilities are poor, in the developed world only 0.1-0.2% of people who suffer the disease will die of it. Nevertheless, many survivors experience long-term symptoms, such as deafness.

In a population where vaccination is sufficiently common, it is impossible for a measles epidemic to take hold; there just aren't enough babies coming into contact with each other to keep passing on the disease, even when adding the rare cases of people where vaccination is unsuccessful or medically problematic.

However, by drastically increasing the pool of potential transmitters, the anti-vaccination movement has ensured that the disease can flourish, particularly in parts of California. Most anti-vaccine campaigners may not be consciously encouraging a disease responsible for hundreds of millions of deaths, but anyone choosing not to protect their own children is also putting the lives of babies they don't know at risk.

Update: The number of cases has now risen to 32, 28 in California and four to tourists who were infected while visiting Disneyland, but not diagnosed before reaching their home state.