I tend to overreact when my grandfather gets sick, even when it is a cold. I don't know why, but I get hyperactive whenever he does. I would go to the pharmacy, sometimes in my pajamas, to get cough syrup, painkillers, et cetera. My grandfather told me to stop getting too worried whenever he gets fevers or colds, but I can't help it.
My mom used to overreact when I would get sick. I was sick as a child and my parents went through hell. Then as an adult if I came down with a cold or flu or anything really my mom would be all over me. I understood it but it drove me crazy.
It sounds like your grandfather getting sick is some kind of trigger to you. Give it some thought and see if you can't come up with a reason why you overreact to your grandfather's health.
You need to make peace with yourself over this. As long as Grandpa takes care of himself and lives a good life, he’s fine. Encourage him to eat healthy and exercise within his limits. Enjoy your time with him and don’t sweat the small stuff.
There is nothing you can do for just about any medical condition (except see a doctor) so I would suggest you read some "How not to Panic" books or see a therapist, if one is available to you.
If your g'pa is otherwise healthy, a cold or headache is not going to do him in. You are really ADDING to his stress when you do this. That will hinder his recuperation. Good luck.