Nine nurses and 3 North Korean physicians involved in a polio vaccination campaign have been murdered in northern Nigeria in recent weeks (http://tinyurl.com/bm9y6jn). No group has claimed responsibility for the attacks, but security experts suspect the radical Islamic sect Boko Haram.
In early February, gunmen killed 9 female health workers and injured 3 others in 2 separate incidents in Kano, the regional capital in the predominantly Muslim north that has the country's largest number of polio cases. These deaths follow the December and January murders in Pakistan of 16 health workers involved in immunizing against polio.
Friedrich MJ. Killings in Nigeria. JAMA. 2013;309(11):1101. doi:10.1001/jama.2013.2400