For this you need to heal someone in your group from terminally ill to slightly sick (ammended from comment).
Starting your own story with 2/3 characters on moderate usually gives you one sick character at the start of the game. This is the more difficult approach as you don't have much medicine/resources at the start.
Alternatively wait until someone gets sick later in the game so you can be better prepared.
Simply don't let them sleep in a bed or give them medicine until they become terminally ill. Stock up on some medicine beforehand so when they become terminally ill you can give them meds each day. Keep them nicely fed and let them sleep in bed during the day and night to bring them back to healthy state quicker.
Bear in mind they will likely become depressed through this process so you might have to talk to them each day to prevent them going broken and possibly committing suicide.