1. It is a customary usage. In my opinion, there no explanation why ? It is like in Chinese you use "一本書". not "一個書". However, you can use "served". Give you another example: do laundry, NOT wash laundry.
2. YES, you can use "before". It means just about the same.
2011-10-24 22:33:35 補充：
If both "until" and "before" are choices, then of course, "until" is better. The reason is when you sue "before', it doesn't specify the time. It means, anytime before a cure is found.
2011-10-24 22:34:20 補充：
However, when you use "until", then it specifies it UP TO a cure is found.
2011-10-24 22:36:55 補充：
Hence, it is better to use "until" between "until" and "before" in this case. However I still like to emphasize that both are correct. I think you ask this question is because of a test. I think this is problem for Taiwan English education.
2011-10-24 22:37:49 補充：
They try to make you distinguish these tiny difference when your feeling with English is not deep yet.