Tai chi or Taiji, does this by improving blood, breath, and lymph circulation. And the flow of Qi. All these things build your immune system, to prevent illness.
Tai chi is also a good system of self-protection, giving you the best chance to handle incidents and accidents by creating time and space for better-trained reactions. It takes time to create time, and space can be found in the body.
Taiji creates harmony for the body, breath, mind and emotions by training relaxation. The more you train relaxation, the better you are at it and the more at ease you are and less dis-ease there is.
