One of the first complaints I heard from other cancer people was that how could they have such a bad thing when they didn't look sick? To this day, I may feel like crap but I don't really look sick.
If you have cancer, you usually don't look sick unless you are in the middle of chemo that gives you that lovely pallor and hair loss. Some chemos don't even give you that look these days. I met a woman a couple of years ago with stage IV lung cancer who looked great and was on an oral chemo. She said she took a daily pill and felt pretty good as well.
We are wired to look for visible ailments in people. Do you ever look at someone getting out of a car with a handicapped placard who looks fine and wonder what is really wrong with them? Maybe its a respiratory ailment or heart condition, but we are suspicious. We expect to see someone with a few medical 'accoutrements' (how's that for sneaking in a big word so early in the day?) such as a cane, walker, etc.
If we hear someone has cancer, we expect to see a pale face with a few straggles of hair peeking out from under a hat or scarf, who is rail thin. But that is less common than the past. If you hang out in the waiting room at the oncology department, you see more people like that but certainly not everyone.
Most people don't realize how many ailments I have. When I had my double ear infection this summer, I was informed I looked pale. When I wear my wrist split for tennis elbow/arthritis, people regularly ask me what's wrong - even total strangers. Maybe I need a full body cast for people to get the point.