Kenyan health officials take the temperatures of passengers arriving at the Nairobi airport on Thursday. Kenya has no reported cases of Ebola, but it's a transportation hub and so is on alert. Simon Maina/AFP/Getty Images hide caption

itoggle caption Simon Maina/AFP/Getty Images

Developing a vaccine for meningitis B was tricky. Even the existing vaccine doesn't protect against all B strains. Josef Muellek/iStockphoto.com hide caption

itoggle caption Josef Muellek/iStockphoto.com