My personal definition of being sick (as opposed to feeling malaise, achey, sniffly, etc.) I first found in one of Suzette Haden Elgin's nonfiction books, and I have liked it enough to take it as my own.

[by the way, her fiction is excellent, creatively addressing issues of men and women and the uses of language--she's a retired professor of linguistics. Try her Ozark trilogy or the Native Tongue series. You'll enjoy them. Promise.]

What she says is particularly apropos for those of us getting older (ahem!).

She says that she knows she's really ill, and should see a doctor, if the activities that usually absorb her and take her mental focus away from herself do not claim her full attention and she keeps returning to how bad she feels.

This criterion has worked for me quite well, as a guideline for deciding when -- or if -- I should consult with a doctor. If I can't lose myself in a really interesting book (a boring book like Vanity Fair doesn't count) or if I can't tend my plants, work in my garden without constantly sitting down, out of breath, thinking how awful I feel...

then it's time to get some medical attention.