Potential of Flavonoids in Viral Infection: From Ethnopharmacology to Clinical Trials

There has been increasing attraction in the discovery and application of new natural flavonoids because of their versatile actions on both plant and mammalian cells. Earlier it was used as nutritious ingredients for humans, later on, the structure-activity relationship was estimated. The total number of hydroxyl groups and the substitution and positions of functional groups on their basic skeleton decide the biological activity and functions of flavonoids. Being plant chemicals, flavonoids cannot be synthesized by mammalian cells. These are widely distributed in all green plants (photosynthesizing cells) except algae and hornworts. An extensive class of naturally occurring polyphenols known as plant flavonoids can be found in many different plant parts, including tea, wine, grains, flowers, fruits, vegetables, grains, and bark. They can be classified in two different ways: (1) on the basis of source and (2) on the carbon of the C ring. They also play an important role in photosynthesizing cells, pigmentation, and pollination of flowers. Approximately, the average balanced diet that contains flavonoids by humans is 100 to 1000 mg per day. Flavonoids have ability for a series of nutraceutical, pharmaceutical, medicinal, and cosmetic applications. They also show a list of biological activities such as anti-viral, antitumor, antioxidant, anti-inflammatory, and so on and acting by several different mechanisms. Quercetin belongs to the flavonol subclass of flavonoids and has more activity than many medicinal plants. Quercetin shows potency against many viruses, also shows antioxidant and antitumor (skin and prostate cancer) activity, and gives positive effects on prostatitis, heart disease, cataracts, allergies, bronchitis, and asthma. So, it can be said that flavonoids may be health-promoting, disease-preventing nutritious natural compounds. This chapter focuses on the structural aspects of flavonoids, their promising roles in human health, and their importance in plants as well as humans.