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 hallar (“to find”) comes from the Latin afflare (“to blow.”) From that same Latin root we get various f-l words involving blowing, including:
All of these share the f-l root. But how did this turn into the Spanish hallar? Well, first remember that the initial F- sound tended to disappear when Latin turned into Spanish; see, fig and higo or fable and hablar. Secondly, note that finding something is just blowing on it, uncovering what was below the dust you blew away!
Herir (Spanish for, “to round”; most commonly heard in the form, “herido”, a wound) is a surprising cousin of… interfere. How so?
Interfere comes to us from the French entre– (“between”) and ferir (“to hit”). Interfering with something is really just hitting it right in the middle of it, breaking it up! Ferir comes from the Latin, for the same, Ferire.
Curiously, Ferire evolved into Spanish Herir — the Initial “F” turning into a “H”. It turns out, this is a common pattern as Latin evolved into Spanish — but in no other language! Just look at Filial and Hijo, or File and Hilo, or Fig and Higo.
Thus, the h-r of herir maps to the (int)-f-r of interfere.
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.
“Fig” comes from the Latin “Ficus” — obvious enough!
But, curiously, the Spanish word is “Higado”. Huh?
This is just a simple example of the Initial F to H pattern. In lots of Latin words, the first F became an H when Latin evolved into Spanish. Think fact/hecho or hablar/fable.
An easy way to figure out what an H- word in Spanish is: change the initial H to an F and see what English word sounds similar.