I think luck has a lot to do with it. Despite being very overweight, I'm probably the healthiest people I know. I'm in my late 50s and the last time I had any time off work sick was when I had my wisdom teeth out about 18 years ago.
In over 40 years of work (including part time jobs when I was at school), the only time I have phoned into work sick unexpectedly was 8 weeks into my first pregnancy when I started bleeding. I didn't want to risk anything, so stayed off work for a few days until I had a scan that showed everything was fine. Other than that, my two pregnancies were trouble free (I was never sick).
I do get mild colds from time to time, but nothing serious, and I haven't yet flu since I was 13.
I don't drink, don't smoke, don't drink tea and coffee, and don't eat much processed food or red meat. I do have a sweet tooth though, and eat too many cakes and biscuits (mainly at work) and chocolate. I work long hours in a stressful job, and do no exercise, except 2 brisk walks of 20 minutes a day as part of my (at least) 3 hour commute.
The only problems I have had in the last few years have been plantar fasciitis (treated by wearing FitFlops), carpal tunnel syndrome (I had a splint for a few weeks) and I think I'm starting to get arthritis in my left hand.
I'm retiring soon, and just hope my luck holds out. The only change I'm planning is to do some exercise to try and get some weight off.