Take cancer: there really isn't a single disease by that name, as far as we can see. There are hundreds (perhaps thousands) of diseases where cells divide uncontrollably and refuse to die off they way that they should. We've given then all the name "cancer", but that's a bit like coming up with a disease called "fever". The biochemical pathways involved can be quite different from case to case, and it's those pathways that our drugs have to deal with. So far, it looks as if we can achieve spectacular successes in narrow fields, where the tumor cells happen to depend on specific factors that we can shut down. But most cancers have multiple weapons firing, and most cancers are (so far) too tough for us. The odds of one drug being able to do the trick are vanishingly long.

The same goes for obesity. Human feeding behavior is complex, and boy, is it well backed up. There are multiple redundant feeding pathways with exta backup redundancy backups, because every species whose members could lose their appetites easily is now dead. Finding a single drug that could thoroughly alter food intake - well, one that does it without making people violently ill, that is - is nontrivial at best, and completely impossible at worst. Considering the number of promising anti-obesity drugs that have crashed terribly, the second possibility is a pretty good bet.

Alzheimer's, though, is perhaps a better one. There may well be a single pathway for the disease, although we're not quite sure what that is. I used to work in this field in the early 1990s, and if you'd told me that we'd still be arguing about the cause of Alzheimer's in 2009, I'm not sure what I would have done. (It would have been reckless.) But if we do figure it out, and it turns out to be something a drug can alter, and we manage to do that - well, that would indeed be a big one. But it's not happening any time soon, unfortunately.

It's a frustrating business, being able to see all these opportunities but not being able to do much about them. The hope has been that research will find a way out, that we'll come up with some great new insights into these conditions which will lead us to therapies that we can't even picture now. Speed the day. It may happen. But it may not, and it may take a long time, no matter what.

