I'm sick.
Why do you think you are sick?
I just think I'm sick but there's nothing I can do about it.
It's good to be cautious if you are sick. It means you will be less likely to spread disease.
It's not like I have chicken-pox or the measles.
You might just have a minor ailment but it's obvious that you don't feel very well. Won't you feel bad if you go around other people and they catch the illness from you?