Fervor is really just an intense passion heating up. Thus we shouldn’t be surprised that it comes from the Latin root fervere (“to boil”), from which we get the Spanish for the same (“to boil”), hervir.
The seemingly unrelated words are connected through the common transformation of Latin words beginning with an f- into an h- in Spanish, such as fig and higo, and fable and hablar.
Thus, the f‑r-v of fervor maps to the h‑r-v of hervir.
The Spanish for “son”, hijo, doesn’t sound like anything in English. But it is a close cousin of the English synonym for brotherliness: filial.
Both come from the Latin for “son,” filius. The transformation to Spanish came about through two interesting patterns: the initial f- in Latin usually turned into an h- in Spanish (such as, hacer and fact, or hablar and fable). The other pattern is less common: the ‑li- sound turned into a ‑j- sound — it’s just a less common sound! Thus the f‑li maps to h‑j almost exactly.
From the Latin filius, we get a few other English words, including: affiliate: an affiliate is, in a way, a child you rear!
From the same root we also get the English fetus, fecund and even feminine. These come, via the Latin filius, from the Proto-Indo-European root *dʰeh₁y-li-os, meaning, “sucker” — in the literal sense of, “one who sucks.” Children, indeed, are defined by their sucking their mothers; so your hijo is literally, “the one who sucks.” And, some might argue, even affiliates themselves usually do suck!
Hervir (Spanish for, “to boil”) comes from the Latin fervere (“to be hot, burn, boil”).
The best part: from this same root, we also get the English… fever!
This is thus another example of the pattern where Spanish lost the initial F and replaced it with the (unspoken) “H”: Hoja-Foliage, Huir-Fugitive, etc.
The English fact comes from the Latin factum, meaning “something that happened.” It is thus an exact cognate to the Spanish hacer, meaning “to make.” How?
The root of both is the Latin facere, meaning “to do.” Fact, and the Latin factum, is just the same word in a different tense.
The Latin facere turned into the Spanish hacer, although they superficially sound different. Their relation becomes obvious once we remember that Latin words that began with an initial f- almost always turned into an initial h- when Latin evolved into Spanish.
Therefore the f‑c-r of facere maps exactly to the h‑c-r of hacer.
This pattern explains many words such as hierro/ferrari, hervir/fever, huir/fugitive, hoja/foliage!
The Spanish hilo (cord; thread; string) comes from the Latin for the same, filum. The words sound very different, until we remember that, words in Latin that began with a f- tended to change to h- in Spanish: hijo/filium, and hoja/foliage, for example. Now the hilo/filum make sense!
Interestingly, however, from that same Latin root filum, we get various English words that also quietly show they are descendants of the word for cord or thread. Including: