More than a billion people live in sub-standard, unhealthy conditions. Unfortunately, most government and charitable organizations are addressing the housing crisis with short term solutions that are unlikely to catch up with population growth. This is also a big concern in metropolitan areas, where slums continue to grow along with the rest of the cities e.g., in Mumbai.
An interesting project out of Princeton, called the Arial Home, looks at a new way of addressing the problem of inadequate housing for poor families. They explore building pre-fabricated cheap homes. A typical home can be assembled by 12 volunteers in a day. More details here.