Well planned nutrition, taking adequate food and belief in natural nutrition and medication, mild Body Exercises like walking, jogging can be some of the things naturally increasing our immunity power. To get rid of pollution we should try to keep our atmosphere clean as far as possible. Pranayama, Yogasanas can also help some extent.
For me, a healthy person should be not only fit physically, but mentally also must be strong. That's make the reason why theory and practical for EQ and IQ established. The same thing applied when strong mind set then of course your self confidence will enhanced.