Dhaka airport looks slightly different as Bill and Melinda Gates's private Bombardier jet touches down in the early-morning heat. Two enormous hand-painted portraits of the couple have been hastily put up next to the runway, and on the road just outside, another huge sign welcomes the world's biggest philanthropists to the capital.

The Gateses are here as part of a four-day trip to Bangladesh and India, where they will be given a firsthand look at what their gargantuan charity, the Bill & Melinda Gates Foundation, is doing in the region, and where it might next direct its attention.