Madhav was an 8th century Indian physician who wrote the Nidāna, which soon assumed a position of authority. In the 79 chapters of this book, he lists diseases along with their causes, symptoms, and complications. He also included a special chapter on smallpox (masūrikā). Ayurveda used a system of inoculation to protect against smallpox. Treatment was done by inoculation with year-old smallpox matter. The inoculators would travel all across India pricking the skin of the arm with a small metal instrument using "variolous matter" taken from pustules produced by the previous year's inoculations. The effectiveness of this system was confirmed by the British doctor J.Z. Holwell based on observations made during his residence in Bengal in an account to the College of Physicians in London in 1767.