California has had 6,400 + cases of whooping cough (pertussis) and 10 infant deaths so far in 2010, which is the highest number of cases they’ve had since the age of immunizations began (about 60 years ago).  Meanwhile, Arizona has had 361 cases and no deaths in 2010 compared to 277 cases in all of 2009.  Even with California’s much higher population, their rate is much higher than ours.  We don’t know exactly why it’s exploding across the river, but it might have something to do with the fact that we require middle-schoolers (who often spread the disease among each other and to little kids) to be vaccinated against whooping cough, California did not.  Late this summer, the California legislature passed a law requiring 7th graders and above to get a pertussis booster starting next year.  Of course it could be alot of other factors too.  As an aside, check out a new article in Science that explains the changing epidemiology of pertussis.