The modern definition of a vaccine (Sp. vacuna) is ‘a preparation of a weakened or killed pathogen, such as a bacterium or virus, or of a portion of the pathogen’s structure, that upon administration stimulates antibody production or cellular immunity against the pathogen but is incapable of causing severe infection’ (AHD). The idea is to stimulate the body’s natural immune system to create antibodies that will fight a disease, typically an infection, though it could also be a non-infectious disease, such as cancer. The purpose is typically to ward off an infection later on and vaccines that do this are known as prophylactic vaccines (Sp. vacunas profilácticas). Other vaccines are given to already infected people to help them fight the disease they already have and are called therapeutic vaccines (Sp. vacunas terapéuticas).
The Spanish name for this preparation, vacuna, gives us a hint of the origin of its name in both languages. In Spanish, the word vacuna was originally a feminine adjective that meant ‘of a cow’ (masc. vacuno) and is related to the word vaca ‘cow’, which is a patrimonial descendant of Lat. vacca ‘cow’. From the noun vacca (vacc‑a), by means of the adjective-forming derivational suffix ‑īn‑, Latin derived an adjective meaning ‘of or related to cows’ whose masculine form was vaccīnus (vacc‑īn‑us) and whose feminine form was vaccīna (vacc‑īn‑a).
So, what do vaccines have to do with cows? To find out we must go back to the late 18th century when Edward Jenner, and English doctor and scientist, described and named in a particular cow disease, known in English as cow-pox and in Latin, as variolae vaccinae. Literally, variolae is the plural of variola, the Late Latin word for small-pox, which was often pluralized, no doubt because of its association with multiple round pox marks it causes (cf. Sp. viruelas). And vaccinae is a genitive feminine form of an adjective that means ‘of the cow’. This cow disease is known as viruela de la vaca or viruela bovina in Spanish (viruela is Spanish for smallpox).
Edward Jenner discovered that cow-pox is related to the smallpox, which affects humans and is quite deadly (cf. §32.3.40 below). He discovered that humans could also be infected by cowpox and that those who did recovered quickly from the disease and, most importantly, developed an immunity to smallpox, a much more lethal disease. The purposeful infection of humans with cowpox in order to give them immunity to smallpox is considered to be the first generation of vaccines, many of which were developed in the 19th and 20th centuries.
Then in the 19th century, French scientist Louis Pasteur was working on the second generation of vaccines for different diseases and he decided to name these biological preparations after the name Jenner gave to the cowpox disease, in his honor. That is because cow-pox is known in French as variole de la vache or just vaccine, after the second part of the Latin name.
The French term vaccine, meaning ‘cow-pox’, was borrowed into English as the name of these prophilactic preparations that were injected in people, pronounced /ˌvæk.ˈsin/. Other languages made their own adaptations to the original term. In Italian and Portuguese, the term is vaccina, by merely changing the French ‑e ending to the feminine ‑a ending native in those languages. Spanish, on the other hand, already had a native adjective derived from vaca, namely vacuno/a ‘of the cow, bovine’, first attested in the 13th century, and it derived the noun vacuna ‘vaccine’ from the feminine form of this adjective. We find the adjective that this noun comes from most commonly in the collocation ganado vacuno, which translates into English as beef cattle. The Spanish adjectival suffix ‑un‑ would seem to be a rare variant of the Latin suffix ‑īn‑, which also turned nouns into adjectives. It is found only in a few words, such as perruno/a ‘canine’ (15th century; < perro ‘dog’). Note that in modern French, vaccine was remade into a masculine noun, namely vaccin /vak.sɛ̃/, which is how you say vaccine today, for Fr. vaccine /vak.ˈsin/ means ‘cowpox’.
In English, from the noun vaccine, the verb vaccinate /ˌvæk.sɪ.ˈneɪ̯t/ was derived, by means of the Latinate verbal suffix ‑ate. And from this verb, the noun vaccination /ˌvæk.sɪ.ˈneɪ̯.ʃən/ was created by means of the noun suffix ‑tion, which is typically added to Latinate English verbs that end in ‑ate (cf. Part II, Chapter 8). This noun can refer to ‘the act of vaccinating’, but it also be a synonym of the noun vaccine, one that is even more common than the original noun, e.g. polio vaccination ‘vacuna contra la polio’. Analogous derivations happened in Spanish. From the noun vacuna (vacun‑a), the verb vacunar ‘to vaccinate’ (vacun-ar) was created and, from this verb, the noun vacunación ‘vaccination’ (vacun-ación). In Spanish, however, the noun vacunación refers merely to ‘the act of vaccinating’ but is not a synonym of the noun vacuna. The transitive Spanish verb vacunar is often used reflexively, i.e. as an intransitive verb. The reflexive verb vacunarse translates as to be/get vaccinated.
 The adjectives Eng. prophylactic ~ Sp. profiláctico/a, meaning something like ‘precautionary’, are loanwords from Greek προϕυλακτικός (prophulaktikos), an adjective made from the following Ancient Greek parts: πρό (pró) ‘before’ and φύλαξις (phúlaxis) ‘a watching, guarding’, a noun derived from the verb φῠλᾰ́σσειν (phulassein) ‘to watch, guard, protect, defend’. The adjectives Eng. therapeutic /ˌθɛ.ɹə.ˈpju.tɪk/ ~ Sp. terapéutico/a /t̪e.ɾa.ˈpeu̯.t̪i.ko/ come from New Latin therapeuticus ‘curing, healing’, from Ancient Greek θεραπευτικός (therapeutikós) ‘attentive, helpful, obliging, curative’. It first appeared in French in the 16th century and from that language it was borrowed into English and Spanish. This adjective was derived from the noun θεραπευτής (therapeutḗs) ‘one who waits on another, an attendant’ (cf. Sp. terapeuta ‘therapist’), which itself is derived from the verb θερᾰπεύειν (therapein) ‘to wait on, attend, serve, cure’, which itself is derived from the noun θερᾰ́πων (therápōn) ‘attendant, aide, etc.’.
 A synonym of the adjective vacuno/a is bovino/a, a cognate of Eng. bovine. These cognates come from Lat. bovīnus/a ‘relating to cattle, from bōs ‘a cow, bull, or ox’ (gen. bŏvis, acc. bŏvem; regular stem: bov‑). Eng. beef is a 14th century loan from Old French buef or boef ‘ox, beef; ox hide’ (modern French: bœuf /ˈbœf/), from Latin bŏvem (accusative form of bōs). Sp. buey ‘ox’ is a patrimonial descendant of the same word bŏvem.
 Other words that have the adjectival suffix ‑un‑ are: moruno/a ‘moorish’ (15th century; < moro ‘Moor’), lebruno (< liebre ‘hare), montuno ‘of the mountain’ (< monte ‘mountain’), bajuno (< bajo/a ‘low’), cerduno ‘of pigs’ (< cerdo ‘pig’), frailuno ‘monkish’ (< fraile ‘monk’), and boyuno (rare) ‘bovine’ (< buey ‘ox’).