Brooklyn, New York is suffering a large outbreak of mumps right now. There have been 600 cases either confirmed or suspected in the past few months. Compare that to the fewer than 300 cases total on average in the entire United States over the course of a whole year and you'll see that this is clearly a major outbreak.
Interestingly, the population affected is overwhelmingly comprised of orthodox Jews. That surprised me; I didn't know of any prohibitions against vaccinations in Jewish culture, and after some research have determined that there is none -- in fact, vaccinations appear to be taken very seriously in Jewish teachings, and there have been times that rabbis have allowed people to get vaccinated even on the Sabbath! However, the report linked above indicates vaccination rates in that area of NYC are lower than the national average, about 80%. It's not clear why.
[UPDATE: I have been informed of an article online that indicates that many of the parents did not vaccinate their children...
...because of their religious beliefs. But local religious leaders said there is nothing in Jewish law that prohibits vaccination. "That's ridiculous," said Rabbi David Eidensohn, a frequent Orthodox Jewish commentator on family issues. "Any parent who doesn't get their child vaccinated is being foolish and endangering the entire community."
A similar outbreak occurred in a Jewish community in Antwerp, Belgium in 2007-2008. In that case, a large number of the parents of unvaccinated children all went to the same physician, who "was opposed to vaccination". Nice work, doc! A vaccination campaign was quickly set up to prevent the outbreak from spreading; together with a high overall rate of vaccination in Antwerp (94%!) this appears to have worked in stemming the tide of the outbreak there.
Most vaccination rates need to be at 90% or higher to provide herd immunity -- where enough people are vaccinated that the potentially infectious disease in question basically doesn't have a place to live. Lower rates mean more people can host the bug, and an outbreak can occur. Worse, the mumps vaccine (usually given together with a measles and rubella vaccination, the so-called MMR shot) has a somewhat lower immunization rate of 80% effectiveness after one dose (that is, 80% of people receiving the shot develop immunity after that one dose -- which is why two doses are recommended). That means herd rates must be higher to prevent outbreaks.
The outbreak in NYC has been traced to an infected child who was visiting the area from the UK. Vaccination rates in the UK are lower, in large part due to the antivax scare started by Andrew Wakefield and his now-discredited study linking vaccines to other illnesses. There have been over 6000 cases of mumps in the UK this year so far (not even including the last quarter of 2009) -- 6000, more than twenty times the cases we get in the US, and we have five times the population here.
All those thousands of kids suffering through mumps -- and potential deafness, encephalitis, meningitis, orchitis in boys, and ovarian swelling in girls -- because of the antivaxxers and their misinformation campaign.
The good news here is that enough people are vaccinated in the Brooklyn area to prevent this from becoming an epidemic. It's clear there would be many more cases if that weren't the case.
As always, don't listen to the lies of the antivax movement. Go to your board-certified physician an ask them about vaccinations, and do the research yourself. Get the truth.