Antonie van Leeuwenhoek, a Dutch draper turned scientist working in Delft, used his hand-crafted microscopes to discover a previously invisible world of microscopic organisms from the 1670s onward. He observed bacteria, protozoa, and eventually the microscopically visible stages of various parasites, opening a new scientific universe and demonstrating that the human body existed in intimate relationship with organisms invisible to the naked eye. His work was communicated to the Royal Society in London and spread rapidly across the European scientific community.

Van Leeuwenhoek's revelations about the microscopic world produced an immediate literary and cultural response: Jonathan Swift's 'On Poetry: a Rhapsody' (1733) famously described the chain of parasites, each being bitten by smaller parasites ('so naturalists observe, a flea / has smaller fleas that on him prey; / And these have smaller still to bite 'em; / And so proceed ad infinitum'). The discovery of the invisible parasite world gave satirists and philosophers a powerful metaphor for human pretension humbled by the infinitely small, and it reshaped the literary imagination of scale and insignificance.