I research food and natural mineral content, and ensure that I have s well balanced diet. I don't smoke, and I don't add sugar to anything, and I try to get enough exercise.
I never get colds or 'flu.
You have the right attitude, as I'm sure you know. These two are absolutely linked, and a lot of people fail to understand it.
Yes, a few people do need supplements and vitamins to help with very specific health issues, but for the majority of people this isn't the case.
So many people see vitamins and supplements as a quick fix alternative to healthy eating, exercise, no smoking, low alcohol... and this is really stupid. If you want to take care of yourself, then diet, exercise etc are the important things. Vitamins and supplements are not a magic quick-fix alternative. A McDonalds plus a vitamin pill does not equal healthy. You can get all the nutrients you need from a balanced diet.
A balanced diet plus exercise, plus cutting back on obvious harms such as smoking and alcohol, and valuing moderation instead of excess - this is all you need. The magic pill doesn't fix things, it's not really magic at all.
I try to have a healthy diet, too. I drink a bit more than I should, but I don't smoke and I do exercise - and I'm someone else who doesn't gets colds or flu.
Basically if you want to be healthy, you have to put a bit of effort into it.