Quercetin is the pigment found in many plant based foods such as cherries, kale, tomatoes, and broccoli. It has been shown to be protective against the first SARS outbreak in 2003. It has also been shown to interefere with the COVID-19 spike protein's interaction with the human cell as well as exhibiting direct antiviral activity. If you are on Hydroxychloroquine, you do not need to take Quercetin. Use caution if you have hypothyroidism.