"Stop/start funding is much more expensive per person reached than a steady flow of funds. Investment in human capital and institutional capacitybuilding can be lost, only to be recreated at great cost when new funding cycles and systems are put in place, often with a different focus." 
These are the words of a contributor to this issue of the Exchange. It is a message repeated many times over around the world. It is a message which will continue to be repeated because quite often people in funding agencies think that they "know about HIV/AIDS"  and they do. But they do not understand the implications of the Scurve - the graph which shows the way in which the number of people infected or sick from HIV/AIDS increases geometrically over time. This fact means that in the face of a long- term epidemic of a slow-acting virus, sustainability - long-term funding, long-term retention of human resources in organizations, long-term relationships with community groups - is of the greatest importance.

Why is sustainability not taken seriously? Lack of money (there is never enough) is one answer. Organizational budget procedures are another: we only plan over three  or two or one  years because our funders cannot plan over a longer period. Fashion is another: "oh, empowerment, participation, gender, poverty, stakeholding... these were popular ideas last month, last year, last decade; funders are now into new Brand X; you'll have to reshape your programme." Behind all these good  and not so good  reasons, what we really need to keep at the forefront of our minds is the Scurve.

We've all seen diagrams and pictures of the virus. These have achieved iconic status. They symbolize something characteristic of the end of the 20th century.

For those not working with medical or clinical aspects of HIV/AIDS, there should be another icon, another symbol, another constant reminder  the Scurve. This represents either the numbers of people infected with HIV over time or the numbers of people with AIDS over time. The latter (people who are sick) follows the former (the number of people infected) at a space of between 2-10 years depending on the rate at which people seroconvert in different places. Like any epidemic curve, it begins slowly and then, when a critical mass of infection has developed, it suddenly rises steeply, accelerating to a peak. We can affect the steepness of the curve and the height of the peak by sustained action. But, for the moment, we cannot alter somebody's serostatus. We can only recognize the need for sustained attention to their welfare and that of their household and community.

The curve of people with HIV infections shows us an image of the future, the numbers of people who will develop AIDS. Together with the S-curve showing the number of AIDS deaths, it reminds us why sustainability is so important in HIV/AIDS work.

We will have to cope with the effects of increased illness and premature death in our societies for many years ahead. In India, estimates show that 3.5-6 million people are  already seropositive; this may indicate a point low down on the Scurve, the early years of the epidemic. These numbers have yet to become people who are ill; the numbers reported to have developed AIDS are very low. In Ukraine and other "transitional" countries, there are small numbers of people living with HIV but the total is rising rapidly. The Scurve provides a reminder of the potential steep gradient and high peak of the epidemic.

We will have to ensure that the rate at which the curve of infection rises is as shallow as possible and that its peak is as low as possible. That's the prevention side. We know that public education, peer education, literacy, taking people out of poverty and changes in the way that people make their livings all can make their contribution. 

But we can neither reduce the gradient of the curve nor lower the point at which it peaks without sustained action. This will be by individuals, households, communities, NGOs, businesses, governments, all contributing in different measure and in different ways, depending on their social, economic, cultural and political environments. Each society's potential for and style of response will reflect its uniqueness. To be sustainable, responses must fit in with all those characteristics that make a society unique   but they must not be bound by them into conservative inaction or illinformed action.

Sustainability takes many forms but the Scurve should remind us constantly of where we want to be and where we might be in 5, 10, 40 years' time.