A reader should check in chapter 17 for solid textual evidence regarding this question. Readers are told right at the start of the chapter that Kit is healing nicely, and she asks Mercy for some help. Kit is shocked to discover that Mercy can't answer because Mercy is hit with a coughing fit that doubles her over. It is almost guaranteed this is not Mercy's first coughing fit, and that means that Mercy's initial reaction to getting sick was to hide it as much as possible from the other members of the family. Other members are sick, and she doesn't want them worrying about her. She holds off as long as she can, but she can't hold out indefinitely. Kit immediately orders her to bed, and Mercy begins crying and protesting because she doesn't want to be seen as weak. We are told that she buries her head in a pillow because she is ashamed of being a burden on the family. As for Mercy's physical response to the illness in general, her body is hit very hard with it. There is definite worry that she won't survive.