Rabies [ˈɹeɪ̯.biz] (Sp. rabia) is ‘a contagious virus disease that affects the central nervous system of warm-blooded animals and is transmitted to humans in saliva, esp. by dog bites, causing throat spasm on swallowing, aversion to water, convulsions, paralysis, and death’ (SOED). The virus in question, Rabies lyssavirus (Sp. virus de la rabia), belongs to the lyssavirus genus of RNA viruses in the family Rhabdoviridae, order Mononegavirales. The term Lyssavirus comes from the Greek λύσσα (lyssa) ‘rage, fury, rabies’ and the Latin word vīrus.
This virus has a wide host range and can be transmitted by saliva and travels to the brain through the peripheral nerves of vertebrates, such as humans and other mammals. Globally, the disease involves mostly dogs, though in the Americas, bats are the main source of infection for humans, since vaccination has greatly reduced the incidence of the disease in dogs. Symptoms start between a month and three months after exposure. They start as a fever and tingling at the place of exposure and they expand to ‘violent movements, uncontrolled excitement, fear of water, an inability to move parts of the body, confusion, and loss of consciousness’ (WP). Once the disease shows symptoms, it is pretty much fatal. The number of deaths by rabies worldwide was around 17,000 in 2015, most of them in Africa and Asia.
The word for this disease comes from the Latin noun răbĭēs, whose primary meaning was ‘madness, rage, fury’, but which could also be used to refer to this disease. That is exactly the same situation that we find with the descendant of this word in Spanish, namely rabia, which can have both meanings: ‘rage, fury’ as well as ‘rabies’. This fifth declension feminine Latin noun changed to rabĭa in Vulgar Latin by adopting the typical feminine ending ‑a, which is the source of the patrimonial Spanish word. English borrowed rabies from Latin in the late 16th century. The noun rabia is most commonly used in Spanish in the expression dar rabia ‘to make furious’, as in Lo que dijo me dio mucha rabia ‘What she said made me furious’ or Me da rabia tener que volver a empezar ‘It makes me furious having to start all over again’. A common expression involving this word is ¡Qué rabia! ‘How infuriating!’ or ‘How upsetting!’.
The Latin noun răbĭēs is derived from the root rab‑ of the intransitive verb răbĕre ‘to be mad, rave’ and the rare derivational suffix ‑ĭēs that formed fifth declension abstract nouns, usually out of adjectives. Two other nouns containing this suffix are pauperĭēs ‘poverty’, from pauper (genitive case: paupĕris; regular root: paupĕr‑) ‘poor, feeble’, from specĭēs ‘external appearance, etc.’, from specĕre ‘to watch, look at, etc.’ (source of Eng. species ~ Sp. especie and Eng. spice ~ Sp. especia).
Spanish does not have a verb derived from Lat. rabĕre, but it does have a very old verb rabiar derived, in Spanish, from the noun rabia. This intransitive verb means something like ‘to rage, be furious’ but only in some contexts and expressions, as in Juan está que rabia ‘Juan is raving furious’. Another common colloquial expression with this verb is a rabiar ‘extremely, awfully, with a vengeance’, which can be used to express an extreme of something, as in La quiere a rabiar ‘He’s madly in love with her’. Also common is the expression hacer rabiar, which is used about typical children’s behavior and is equivalent to Eng. to tease or to make mad, as in Siempre le hace rabiar a su hermanito ‘He’s always teasing his little brother/making him mad’.
Another name for rabies is hydrophobia (Sp. hidrofobia), which is the disease’s historical name since aversion to water and inability to swallow is one of the symptoms of rabies in humans. The term hydrophobia means ‘extreme or irrational fear of water, especially as a symptom of rabies’ (COED). It is a loanword from Latin hydrophobia, itself a loanword from Ancient Greek ὑδροφοβία (hudrophobía), formed from the combining form ὑδρ‑ of Greek ὕδωρ (húdōr) ‘water’ and the suffixoid -phobia ‘fear, aversion’ derived from Ancient Greek φόβος (phóbos) ‘fear’ (ὑδρ‑ο‑φοβ‑ία). To differentiate the ‘rabies’ sense of this word from the simple ‘irrational fear of water (and of drowning)’ sense, the term aquaphobia has been coined for the latter sense, which is formed from the Latin word for ‘water’, aqua (cf. patrimonial Sp. agua) and the same suffixoid (aqua‑phobia).
There are a couple more English words derived from the Latin root rab‑. The adjective rabid [ˈɹæb.ɪd]. It means ‘relating to or affected with rabies’ but, also, ‘extreme; fanatical’ (COED), as in a group of rabid right-wing fanatics (DOCE). It is an early 17th century loan from Lat. răbĭdus ‘raving, furious, enraged, savage, fierce, mad, rabid’ (L&S). This Latin word has not made it into Spanish, but another synonymous Latin adjective has, namely rabioso/a, first attested in the 13th century. It comes from the less common Latin adjective răbĭōsus ‘raving, fierce, mad, rabid’, formed with the adjectival suffix ‑ōs‑ (răbĭ‑ōs‑us; fem. răbĭōsa).
Finally, the English noun rage [ˈɹeɪ̯ʤ], whose main meaning is ‘ violent uncontrollable anger’ (COED), can be traced to the Vulgar Latin noun rabia. In came into English in the 14th century through Old French rage or raige ‘fury, madness, etc.’, which is first attested in the 11th century and comes from Vulgar Latin rabia. The verb to rage is attested in English even earlier, in the 13th century, originally meaning ‘to go mad; to be mad; to act madly or foolishly’. Today, this intransitive verb means ‘to feel or express rage’ (COED). This verb came from the Old French verb ragier (Modern French rager [ʀa.ˈʒe] is used colloquially with the meaning ‘to fume, be raging mad’). From this verb, Old French derived the verb enragier by prefixation, meaning ‘go wild, go mad, etc.’, which was borrowed into English in the late 14th century as to enrage, a transitive verb that today means ‘to make very angry’ (COED). The adjective enraged derived from this verb’s identical past participle is actually more common than the verb.
 The word for rabies in Ancient Greek was λύσσα (lússa) which, as in Latin, also meant ‘rage, fierceness, fury’. In Greek mythology, Lyssa (Λύσσα) was ‘the spirit of mad rage, frenzy, and rabies in animals’, a personalization of these emotions. (The name was Lytta Λύττα in the Athenian dialect of Greek.) This personification of rage was closely related to the Maniae, which were were the personified spirits (daimones) of madness, insanity, and crazed frenzy (Sp. manías). The Roman equivalent of this spirit was known variously as Ira, Furor, or Rabies.
 This is analogous to the change from Lat. diēs ‘day’ to Sp. día, though Lat. diēs was typically masculine, as is Sp. día. It was feminine only when personified as a goddess.