[1] His paternal family had been prominent Leiden citizens since the 15th century, carrying a coat of arms with three violins, which Van Swieten modified and adopted when he was made a Baron in 1753.

[6] Back in Leiden he enrolled at the University again and became a resident pupil with the pharmacist Nicolaas Stam, whose father had introduced Herman Boerhaave to chemistry.

[7] With Albinus as his advisor, he obtained his medical doctorate in July 1725 with a dissertation on the structure and function of arteries (De arteriae fabrica et efficacia in corpore humano).

[9] He saw many patients and soon, apparently with Boerhaave's permission, also started giving private lessons in pharmacy and materia medicae, drawing 60 British students for his first course alone.

[14][15] November 1742 saw the death of Joannes Baptista Bassand, personal physician of the Holy Roman Empress Maria Theresia and a former student of Boerhaave.

As a Catholic and a foremost student of Boerhaave, Van Swieten received an offer to fill this position as well as that of director of the court library in early 1743.

He respectfully declined, writing to a friend that he much preferred to remain "a small republican than to carry a pompous title that simply conceals a slavish existence".

His report, Abhandlung des Daseyns der Gespenster (or Discourse on the Existence of Ghosts), offered an entirely natural explanation for the belief in vampires.

Characteristic for his opinion is this quotation from the preface to his essay of 1768 "that all the fuss doesn't come from anything other than vain fear, superstitious credulity, dark and eventful imagination and simplicity and ignorance among these people."

In response to the report, Maria Theresa issued a decree that banned all traditional defences that locals had been using, such as putting accused vampires to the stakes, beheading or burning them.

In 1758, he also examined and treated Magda Logomer, a woman condemned to death in Zagreb for witchcraft at the request of Maria Theresa, leading to the verdict being cancelled by the queen, ending a phase of witch trials in Croatia.