Jan (or John) Ingenhousz or Ingen-Housz FRS (8 December 1730 – 7 September 1799) was a Dutch-born British physiologist, biologist and chemist. He is best known for discovering photosynthesis by showing that light is essential to the process by which green plants absorb carbon dioxide and release oxygen. He also discovered that plants, like animals, have cellular respiration. In his lifetime he was known for successfully inoculating the members of the Habsburg family in Vienna against smallpox in 1768 and subsequently being the private counsellor and personal physician to the Austrian Empress Maria Theresa. Early life He was born into the patrician Ingen Housz family in Breda in Staats-Brabant in the Dutch Republic. From the age of 16, Ingenhousz studied medicine at the University of Leuven, the Protestant Universities were not then open to Catholics like himself, where he obtained his MD in 1753. He studied two more years at the University of Leiden, where he attended lectures by, among others, Pieter van Musschenbroek, which led Ingenhousz to have a lifelong interest in electricity. In 1755 he returned home to Breda, where he started a general medical practice. Work with smallpox Following his father's death in July 1764, Ingenhousz intended to travel through Europe for study, starting in England where he wanted to learn the latest techniques in inoculation against smallpox. Via the physician John Pringle, who had been a family friend since the 1740s, he quickly made many valuable contacts in London, and in due time became a master inoculator. In 1767, he inoculated 700 village people in a successful effort to combat an epidemic in Hertfordshire. In 1768, Empress Maria Theresa read a letter by Pringle on the success in the fight against smallpox in England, whereas in the Austrian Empire the medical establishment vehemently opposed inoculations. She decided to have her own family inoculated first (a cousin had already died), and requested help via the English royal house. On Pringle's recommendation, Ingenhousz was selected and requested to travel to Austria. He had planned to inoculate the Royal Family by pricking them with a needle and thread that were coated with smallpox germs taken from the pus of a smallpox-infected person. The idea of the inoculation was that by giving a few germs to a healthy body the body would develop immunisation from smallpox. The inoculation was a success and he became Maria Theresa's court physician. He settled