Balmis conducted experimental trials on the effectiveness of two Mexican plants, agave and begonia, which were believed, according to folk medicine practices in Mexico, to cure syphilis and scrofula. The trials confirmed that the plants were ineffective. He published the results in a deluxe book with two handsome hand-colored plates.

On November 30, 1803 Spanish physician Francisco Javier de Balmis and his team embarked from Spain, on an expedition to vaccinate the people of Spanish America against smallpox. This three year voyage, which became known as the Balmis Expedition, is considered the first international health care expedition. Of it Edward Jenner wrote, " I don’t imagine the annals of history furnish an example of philanthropy so noble, so extensive as this." On the ship Maria Pita Balmis sailed with a deputy surgeon, two assistants, two first-aid practitioners, three nurses, Isabel López de Gandalia, the rectoress of Casa de Expósitos, an orphanage in La Coruña, and 22 orphan boys, eight to ten years old, who served as successive carriers of the disease. The mission carried the vaccine to the Canary Islands, Colombia, Ecuador, Peru, Mexico, the Philippines and China. The ship carried also scientific instruments and copies of Balmis's translation into Spanish of Traité historique et pratique de la vaccine (1801) by Jacques-Louis Moreau de la Sarthe. Digital facsimile from the Internet Archive at this link.