Hi Helen,
Good question. The discussion mainly depends on the character of the particular health issue. So one could say “There is a pain in …”, “I am feeling sick.”, or “I might have a temperature.” or anything like this. There are tons of expressions in respect to health issues possible. Are you interested in the vocabulary in respect to a special illness or treatment?
Perhaps the best way to answer someone who is feeling under the weather is to wish them “Get well soon!”.
Best regards,
Paul