He was the second of four sons of a Protestant clergyman, all of whom became professional men, his brother Adair being physician to St. Thomas' Hospital, London, and professor of chemistry at Woolwich.

In 1790 he received from the Dutch government the appointment of surgeon-major to the colony of Demerara in South America; there he had charge of a military hospital of sixty to eighty beds.

He held high rank in his profession, being censor, examiner, orator, and member of the committee to publish the Transactions of the Medical and Chirurgical Faculty, and consulting physician to the Board of Health and City Hospital.

He ransacked the whole realm of nature and brought together a great mass of evidence to prove this theory which he maintained, notwithstanding its unpopularity and prejudice to his professional success, with all the ardor of absolute conviction.

He argued convincingly from the known to the unknown, and declared prophetically that while the minute animalcula could not then be demonstrated (in fact, the necessary microscopes already existed, but were expensive, and systematic research was lacking), they are not beyond the reach of human ken and in due time would be recognized.

The bigotry and prejudices of his contemporaries compelled him to publish his opinion in a non medical periodical, The Baltimore Observer, in which they appeared in 1806 and 1807 under the heading “Quarantine.” We may conclude that John Crawford made an independent discovery of this theory, and so far as was known to Eugene Cordell in 1920 he was named the first in all history who investigated it in a thorough and scientific manner.