Microsoft tycoon Bill Gates on why vaccines are magic

Microsoft tycoon Bill Gates has pledged $1bn to help vaccinate children around the world against preventable diseases like pneumonia.

He is hosting a summit in London where countries are being asked to give an extra £2.3bn ($3.7bn) by 2015 for child vaccines.

The Global Alliance on Vaccines and Immunisation (Gavi) says this could help save four million lives in four years.

Gavi is a health partnership of governments, businesses, and bodies including the World Bank, Unicef and Mr Gates' personal foundation.

The BBC's medical correspondent, Fergus Walsh, was at the summit and spoke to Mr Gates.