Having a genetic disease means that there's something wrong with your DNA. Somewhere, in those millions of base pairs, even the simplest mutation (or mis-coding) in a gene can cause all sorts of serious problems, and since the problem is at such a basic level, it's impossible to fix without rewriting the essence of what makes you you. And we can now do that.
You can think of a genetic disorder like having a bug in the firmware of a computer system. Generally, it's hard for end users to fix problems like that, because we typically can't access or modify our computers on such a low level. All we can do is mess with software to try and mitigate whatever damage the firmware bug is causing. So maybe you won't run certain programs because they tend to cause your system to crash, or you install a bunch of software to try and work around the bug. In many cases, you can make things substantially better by doing this, but you can't solve the problem, since it's firmware and not software.
The difference between this computer analogy and the reality of a genetic disease is that with a little bit of work, you can apply a firmware patch to solve firmware-related issues. And now there's a way to do that in humans, too: we're starting to get to the point where we can send in a little program, in the form of a drug, that will rewrite the operating system that's been hard-coded into our DNA to fix genetic errors, and the first Western medication that can do this has just been approved for use in Europe.
Glybera is a drug that's designed to treat a genetic disorder called lipoprotein lipase deficiency (LPLD). You've probably never heard of it, but about 1,000,000 people around the world suffer from it. LPLD is caused by a mutated gene that prevents people from making the enzymes necessary to properly metabolize fatty acids in their blood, leading to severe abdominal pain, diarrhea, and acute (and sometimes fatal) pancreatitis. It sucks. And all people with LPLD have been able to really do is try to eat as little fat as possible. Glybera, though, actually goes in and modifies your DNA directly, using a deactivated virus to deliver new genetic code into cells, teaching them to properly produce the fat-metabolizing enzymes that they've been missing. It's not a treatment, it's a cure.
For most of us, what's most exciting about Glybera is not the fact that it can treat LPLD. The exciting part is that Glybera has been approved for use outside of a research hospital or clinical trial, and that anyone who needs it will be able to use it as of 2013. This is just the first of what we hope will be an entirely new generation of drugs that can treat diseases that were, until now, untreatable. For example, the largest study of using gene therapy to treat cystic fibrosis ever is currently taking place in London, and there's promising research into a wide variety of other genetic disorders, including hemophilia and a disease that causes blindness in kids.
Like any cutting-edge new drug, Glybera isn't cheap: it's a series of one-time injections into a patient's leg muscles, and will cost insurance companies in Europe $1.6 million (!) per dose. But scientific progress like this is inherently expensive, and when measured against the potential for improving the lives of millions and millions of people who suffer from genetic diseases (and all of the people who are friends and relatives of those people), there's absolutely no question that it's worth it.