http://www.forbes.com/sites/tarahaelle/2015/01/20/five-things-to-know-about-the-disneyland-measles-outbreak/

1) Two things are driving the spread of measles – and neither is undocumented immigrants.

Those two things are the extreme infectiousness of the disease and the low levels of herd immunity, or community immunity, in pockets of southern California. Measles infects 9 out of every 10 non-immune individuals it finds. It’s airborne and hangs around up to two hours after an infected person leaves the area. It doesn’t take much for this disease to spread through a population that isn’t immune from previous exposure or through vaccination. Or, to put it another way, in an unvaccinated population, each person infected with the measles will transmit the disease to 12 to 18 other people. If no one were vaccinated against measles, we would be up to hundreds, perhaps thousands, of cases by now. We aren’t because there are some levels of herd immunity, but it’s because herd immunity has been weakened that we’re seeing additional cases at all.

Meanwhile, one of the biggest myths popping up in comment threads and on social media is that undocumented immigrants have something to do with this outbreak, or any other outbreak of a vaccine-preventable disease. We don’t yet know who Patient 0 – the first person with the disease – was at Disneyland, but we don’t really need to know. It’s not undocumented immigrants we should be pointing the finger at. It’s home-grown, upper-middle class, well-educated, mostly white southern California parents who have chosen not to vaccinate their children we should be giving the side-eye to. When vaccination rates in the region are below some developing countries’ rates, you don’t need undocumented immigrants to bring in the disease. Unvaccinated Americans do a fine job of that all on their own. A look at past cases makes this clear.