Shabbat 109 - Healing on Shabbat
One may not prepare brine (salt water for pickling) in large quantities on Shabbat, because he is in fact preparing for the weekdays after Shabbat. However, one may prepare a small quantity that will be consumed that day. Rabbi Yose challenged this: "Is brine not the same, whether in large or in small quantities? If you have a law like this one, then people will say that a small amount of work on Shabbat is permitted, and only a large amount is forbidden. Rather, all brine is forbidden to be made on Shabbat, unless one first adds oil either to water or to salt - and in this way he makes a distinction and a reminder that it is Shabbat."
Generally it is prohibited to effect cure of minor diseases on Shabbat. The Sages accepted this decree due to the possibility that one may come to grind medicine on Shabbat, which would definitely be prohibited. However, if he eats foods that have therapeutic effect, but that are generally eaten also by healthy people, then it is not obvious that he does it for healing, and it is allowed.
Art: Gabriel Metsu - The Sick Child