According to the most famous story in Hinduism, Dhanvantari was born during the churning of the ocean (Samudra Manthan) on the Trayodashi Tithi of Kartik. He is considered an incarnation (avatar) of Lord Vishnu, who took the form of Dhanvantari to spread the knowledge of science and medicine on Earth. Therefore, the day of Dhanvantari’s appearance is celebrated as Dhanvantari Jayanti.
When the ocean was churned for nectar, Dhanvantari emerged from
the ocean holding a pot of Amrita (nectar). Bhagavan Vishnu said to him, “You
were born from the waters, so you shall be called ‘Abja’,” meaning born of
water.
Dhanvantari then requested Vishnu to grant him a share in
sacrificial offerings (Yajna).
Vishnu replied that the divisions of the Yajna had already
been assigned to gods and demons, and since Dhanvantari was born after the
Vedas were established, no mantras existed for his invocation in rituals.
However, Vishnu granted him a boon—to take a second
incarnation in the Dvapara Yuga.
In that incarnation, while still in the mother’s womb,
Dhanvantari would gain divine powers and attain godhood. He would be the one to
popularize Ayurveda, and would be worshipped by people, with mantras, ghee, and
offerings from cows.
During the Dvapara Yuga, the King of Kashi (Varanasi), after
long penance desiring a son, worshipped Dhanvantari. He agreed to be born as
his son.
Dhanvantari became a great king, known as Sarvarogahari — “the
remover of all diseases.” The sage Bharadvaja taught him the science of Ayurveda,
making him the founder of medical science. Dhanvantari classified medical
knowledge into eight branches and spread it among his disciples.