Achilles was the Trojan War hero, son of the goddess Thetis, who tried to make him impervious to any attack by dipping him in the River Styx when he was a baby. For this process, she held him by the heel - so his heel didn't get wet. Achilles grew to be a great warrior, but was eventually killed when Paris shot him in the heel with an arrow.
Unlike Achilles, I'm not impervious to attacks. I tend to suffer constantly from aches and pains, and because I take drugs to suppress my immune system I pretty much catch every but that's going around. I don't have just one vulnerable area. I've vulnerable pretty much everywhere.
If there is one area that is worst of all for me, it is - and I know this sounds strange - the times when I forget that I'm sick. How could I forget, when I swallow pills by the bucketload? Simple. I have a few good days, or even a few good weeks. I do a bit more than I have been doing, and get away with it. I'm really well (for me at least.) So then I do a bit more. Then I skip my afternoon nap, because, after all, I'm really well at the moment. Then I skip my afternoon nap for a couple of days in a row. Then I do a bit more exercise than usual, a bit more work than usual, then a bit more housework than usual.....
Well, you can see where it's going. There's a limit to how much "extra" I can do. My rheumatologist has a few standard lectures he gives me regularly, things like: "with lupus, if you over-use it, you lose it" and "stop trying to prove you can have a normal life."
I don't really try to prove anything. I just feel so well, that I just do what I feel I'm able to do. But feeling can be deceptive. I can get away with doing just a bit more for a while, but eventually my body says, "that's enough", and suddenly I'm overwhelmed with pain and fatigue and it will take days and days of rest for me to start feeling human again!