You should take some medicine.
Correct!
You look very sick. Why do you go to the doctor?
Incorrect: Why don't you go to the doctor?
I am having a sore throat.
Incorrect: I have a sore throat.
My ear hurt. What should I do?
Incorrect: My ear hurts. What should I do?
Lisa and Tina both has coughs.
Incorrect: Lisa and Tina both have coughs.
What wrong?
Incorrect: What's wrong?
I have a headache.
Correct
I have stomachaches.
Incorrect: I have a stomachache.