Not just the D3 but because I eat a very healthy diet (no junk and everything organic) and I'm a fitness fanatic/age-group athlete, I trust my immune system to effectively fight off viral & bacterial infections. And that's exactly what my immune did when my son & I were stricken with a case of the severe flu last Feb, 2020.
As a baby boomer, I've also had all those childhood viruses (mumps, measles, chickenpox, etc.) and I'm still here. Lol. And several years sgo, I took the flu shot and got real ill afterwards - so no more of that. I just don't fear these viruses like many other people do. You be surprised how healthy eating and regular excercise can go a long way in keeping your immune system healthy.
It would depend on what kind of cancer and what type of treatments are recommended. And on the cardio issues, definitely not worried. A few years back I had a treadmill test and some other screening. Came out with a cardio fitness level that was off the charts for my age (55 at the time) and comparable to a fit 25 yr old. I'm currently doing Silver Era Bodybuilding training (1940s/50s style) and some running & triathlons. 7 days a week training, healthy eating, good sleep & supplements works for me.
And at the gym, I've got other friends & acquaintances, young & old, male & female, conservatives & liberals who are also fitness fanatics and eat healthy who are not interested in taking the vaccine either.
I can 'me too' to all of your first stuff. I've eaten healthy all of my life because my parents were 'health food nuts' so thankfully to them I've always eaten well. If you'll recall, I have talked about how eating a balanced diet is the key to health, not mega dosing supplements. I have also been an athlete my entire life, including being a pro mountain bike racer for many years, and I still ride about 250 days a year with hiking, jogging, weights tossed in for some variety so my fitness is not too shabby. I too have a very strong immune system , and
likely wouldn't get very sick from COVID, but I also want to protect those around me (family, friends, colleagues).
"You be surprised how healthy eating and regular exercise can go a long way in keeping your immune system healthy." Yes, because you are the only one on the forum who knows this.
Your second bit...head shaking. You know more than the oncologists about how to treat cancer? What IF you do have cardio issues? Are you going to megadose your way out of it. You don't worry about cardio issues? That's very uniformed of you because while fitness may help, it doesn't change genetics. Look at the healthy young people, and 40 year old elite athletes who have cardio issues. Does Silver Era Training prevent heart valve issues?
I hope that you don't get sick with COVID, cancer, cardio, or pass COVID to others, but your way of thinking (about this anyway) is self centered and arrogant.
EDIT: credit where credit is due: I'm glad that you wear a mask to limit the spread if you do get COVID.
EDIT2: its interesting that you're so confident in your immune system when a garden variety influenza knocked you out last year (two weeks right?) and as you said then "took all of your fitness".