If your sickness is one from which there is the hope of recovery, then after you recover you have to make up the fasts that you missed. But if your sickness is permanent and there is no hope of recovery, then you have to feed one poor person for each day that you did not fast.

It is permissible for pregnant women not to fast in Ramadan. All they have to do is to make up the days they missed, whether they feared for themselves or their child.

The majority of scholars view that fasting whilst traveling is permissible. If the traveler fasts, it counts and he does not have to make it up. For more, please see the detailed answer