The greatest challenge for the 21st century in my field, and a major challenge for all of us on the planet, is preparing for and responding to the great viral pandemics. There is the pandemic which we have today, HIV/AIDS. We have known about it for 25 years. We have had little impact on its relentless expansion. It has already killed 25 million people worldwide and is infecting three million more every year. It will get much worse before it gets better even if we did all the right things tomorrow (and there is little chance of that). Then there is the big new scary viral pandemic that we fear - pandemic flu. It will spread rapidly throughout the world. It could kill between 100 and 300 million people. During the period of maximum panic, it would paralyse all normal economic activity and create huge economic losses (estimated at roughly five per cent of total global GNP).
Preparing for these events (we are currently not adequately prepared and our defences are weak) and responding to them when they occur (our responses are currently feeble and disorganized) is among the highest priorities for humankind in the coming decades.