The adjectives fervent and fervid both mean having great passion or zeal, but fervid is more extreme. One who is fervent is merely passionate; one who is fervid is passionate to an extreme, perhaps irrational degree.
Both words come from the Latin fervere, meaning to boil, and both may still mean extremely hot, glowing, or boiling, but this sense of fervid and fervent is obsolescent and rarely appears in modern writing.
Examples
Although we draw a distinction between fervent and fervid, the words are usually used interchangeably—for example:
Gen MacArthur was a fervent anti-communist, so his argument made a strong impression on Kennedy. [Financial Times]
… and the fervid knee-jerk criticism is something that’s relevant to the real world. [AV Club]
This fervid work is by turns sombre and reflective, then demonstrative and dramatic. [Vancouver Sun]
As a fervent anti-recliner, it’s embarrassing and an annoyance to have to spend the whole flight pulling the seat upright … [Sydney Morning Herald]
