It depends on the context. Is this about work, meeting for a movie, feeling sick while together doing something, two people who just met on the subway, etc? No one can answer this without knowing the context. 1 to 4 are all good normal answers depending on the situation and so are Amber's suggestions. Rene's answers are strange and not even true except in very special situations (I notice he is not a native speaker, but it seems like he is giving you a bad answer on purpose) However, the context must be known. So there is no way to give you the correct answer

1). Really. "I don't quite believe you, but neither do I probably care." 2) take care then. "Whatever, get better, but I don't really care." 3) ok. "I don't care" 4) hmm. "I don't care (even what you said)" :D

Here are some appropriate responses if someone tells you they are sick: - I'm so sorry. - I hope you feel better. - Get well soon! -Do you need any help?