There is a lot about COVID 19 that is yet to be understood. What is known is that the virus leads to pneumonia, damages lung tissue, leading to impaired breathing. It is also noted that the virus symptoms are more pronounced in older people than in younger people. There have been cases of severe infection and mortality, though, in young people too.
Pranayama group of breathing exercises help improve lung functioning. To that extent, theoretically, this should be of help in building protection against any infection. Similarly, fruits and vegetables that are rich in antioxidants are known to build body immunity.
In Chinese medicine, tai chi and qi gong are also known to help build body strength, bring calmness to mind, and build immunity. Scientific studies have validated these properties of yoga, meditation, tai chi, and qi gong.
There are several herbs and herbal combinations that have been known to strengthen immunity and improve body vitality. Some of the plants that immediately come to my mind are turmeric, ashwagandha (Withania somnifera), Indian gooseberry (Embilica officinalis), holy basil (Ocimum tenuiflorum), and many others. These are ingredients in many of the Ayurveda nutraceutical combinations. These combinations are anecdotally seen to help build body immunity. Individually scientific studies have validated immunity building and other medicinal properties of these plants.
Most of these benefits do not have scientific validation via human clinical trials. Therefore, it is best to seek a professional medical opinion before self-medicating with these practices and herbs.
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