|Maciej Czyżewski, CC-BY-SA-3.0,2.5,2.0,1.0, via Wikimedia Commons|
"Zombie bees" (aka zombees) are showing up is some parts of the United States. Don't worry, they aren't coming after our brains. The poor critters are infested with a parasite.
Apocephalus borealis, a fly known as the "scuttle fly" or "zombie fly", is the culprit. This insect has been known to infect bumble bees and paper wasps in the past, but now it is also using honey bees as hosts. The adult fly lands on the bee's back and injects the eggs into the abdomen. The eggs hatch, and the maggots eat the bee from the inside while they grow. Once a bee is infected, it will abandon the hive at night and fly around erratically in movements reminiscent of a zombie. This goes on until the bee dies and the maggots crawl out to pupate.
As of October 3, 2012, the problem is primarily in California, where the bees were first discovered in 2008. However, confirmed cases of infection have also been confirmed in Oregon and Washington State, and as far east as South Dakota. The flies themselves have been found scattered across the United States and Canada, all the way to the east coast.
You can find updates on the spread of the infection at ZomBee Watch.