An infectious disease is one that involves ‘the invasion of an organism’s body tissues by disease-causing agents, their multiplication, and the reaction of host tissues to these organisms and the toxins they produce’ (WP). There are many types of infections, from the mild common cold to some that are quite lethal. Approximately 17% of all deaths are caused by infectious diseases nowadays.
An infection is typically communicated from another organism, such as another human, and thus, infectious disease is also known as a contagious disease, also known as a communicable disease, a transmissible disease. The Spanish equivalent of these terms is enfermedad contagiosa.
There are many types of agents of infection, also known as germs. The best-known ones are viruses and bacteria, but there are others, such as viroids, prions, nematodes (such as parasitic roundworms and pinworms), arthropods (such as ticks, mites, fleas, and lice), fungi (such as ringworm), and other macroparasites (such as tapeworms and other helminths) (WP).
Some of the most common infectious diseases are influenza (flu, grippe), measles, cholera, diphtheria, smallpox, malaria, meningitis, pneumonia (many different causes), strep throat, typhoid (fever), tuberculosis, HIV/AIDS, hepatitis, malaria, yellow fever, and various sexually transmitted diseases (STDs).
Finally, modern medicine has developed a number of medications to deal with infections, such as antibiotics, antivirals, antifungals, antiprotozoals, and antihelminthics.
In this chapter, we are going to look at some of the vocabulary used to talk about infectious diseases in English and Spanish. We will find that there are many cognates among these words.
Eng. disease, Sp. enfermedad, and related terms
Eng. disease and Sp. enfermedad are equivalent, but they are not related. They do have one thing in common, however, namely that both words had a negative prefix when they were first put together, even though speakers do not have a way of sensing that negative prefix any longer in the modern versions of these words.
Eng. disease was originally literally the opposite of ease, to which the negative Latinate prefix dis‑ was added. And Sp. enfermo ‘sick’, the source of enfermedad ‘illness’, comes from Lat. infirmus, with the negative prefix in‑ (cognate of Eng. un‑).
Eng. disease, pronounced /dɪ.ˈziz/ or /də.ˈziz/, just like the word ease, which was originally its opposite, are loanwords from Old French. The word ease /ˈiz/ entered English by the early 13th century, from Old French aise, which presumably meant originally ‘elbow room, side space/room’, and, derived from it, ‘comfort’ and ‘well-being’. It is not clear, however, what the ultimate source of this French word was. (Modern French aise /ˈɛz/ means ‘delighted’ as an adjective and ‘pleasure, joy’ as a noun.)
As for the opposite of ease, disease, it was borrowed by the early 14th century from Old French desaise, with the primary meaning ‘discomfort, inconvenience’, etc. (it was spelled disese in Middle English). It was formed, in French, by adding the Latinate French prefix des‑ ‘without, away’. This prefix is a descendant of Lat. dis‑ ‘apart, not, etc.’, which has been borrowed as a prefix in English words, such as disentangle and discount, as well as being found in many loanwords, such as disease.
By the late 14th century, Eng. disease already had its modern meaning of ‘sickness, illness’, though the older, original sense survived until the early 17th century. The word disease came to replace two native Germanic, English words for this meaning, namely adle, from Old English ādl, and cothe /ˈkoʊ̯ð/ from Old English coþu, both meaning ‘disease, illness’.
As for the noun enfermedad, it comes from Latin īnfĭrmĭtās ‘weakness’ (genitive: īnfirmĭtātis; regular stem: īnfirmĭtāt‑; morphemes: īn‑firm‑ĭ‑tāt‑). This noun was derived from the adjective ĭnfĭrmus ‘weak, feeble’ (fem. īnfirma), which as we mentioned earlier, comes from fĭrmus ‘firm (not frail), strong, stable’ (Vulgar Latin fīrmis), the source of Eng. firm and Sp. firme (cf. Fr. infirme, It. infermo, infirmo, Port. enfermo). Sp. enfermo ‘sick, ill’ is a semi-learned descendant of ĭnfĭrmus ‘weak’. It seems that by the time of Old Spanish, the word had already changed meaning from ‘weak’ (Sp. débil) to ‘sick’, which is a semantic change that is easy to explain.
English has cognates of both enfermo and enfermedad, namely infirm and infirmity, though they are fancy words and their meaning is not the same as their Spanish counterparts, since the original sense of ‘weakness’, not ‘illness’ is at their core. Thus, infirm means ‘not physically strong, especially through age’ (COED). Some dictionary definitions do mention, however, that infirmity could be caused by disease.
Spanish words derived from enfermo in Spanish, besides enfermedad, include enfermero/a ‘nurse’, enfermería ‘infirmary, sick bay; nursing (career)’, enfermizo/a ‘sickly’, and enfermar or enfermarse ‘to get sick’.
Synonyms of Eng. disease
Eng. disease has two synonyms, sickness and illness, which are not exactly identical, though they are often used interchangeably. All three words translate into Spanish as enfermedad, however.
The word disease is the most objective of the three and thus the one typically used in medicine. It can be defined as ‘a pathological condition of a part, organ, or system of an organism resulting from various causes, such as infection, genetic defect, or environmental stress, and characterized by an identifiable group of signs or symptoms’ (AHD). The word illness, on the other hand, refers to what can be the same thing but from a patient’s perspective. Note that a person may have a disease without feeling ill, and vice versa.
The noun illness /ˈɪl.nəs/ is derived from the word ill /ˈɪl/, whose main meaning is ‘not in full health; unwell’ (COED), by adding the suffix ‑ness that creates nouns ‘denoting a state or condition’ (COED). It is a loan from Old Norse that originally meant something closer to ‘evil’. The noun sickness /ˈsɪk.nəs/ derives from a native English adjective sick /ˈsɪk/. The same dictionary defines illness as ‘poor health resulting from disease of body or mind; sickness’, as well as ‘a disease’, and sickness as ‘the condition of being sick; illness’, as well as ‘a particular type of illness or disease’. As we can see, the two definitions are rather circular.
As for the words they are derived from, we should mention that ill is used primarily as an predicative adjective, equivalent to sick, as in to be ill, or as an adverb, equivalent to ‘in an ill manner, badly’ (OED), as in to speak ill. On the other hand, sick can be used as an attributive adjective, as in to be sick, or else as an attributive one, as in a sick person (notice that we do not say *an ill person).
One difference in the use of these words is that disease is, as we have seen, a more technical and objective word, so it is more likely to be used by the medical establishment. Another difference in usage is that the word disease is used preferably for cases that are more life threatening or that require serious treatment.
There is yet another synonym in English of the three words that we just discussed, namely ailment. This word refers to ‘a minor illness’ (COED). Thus, it is not surprising that, for emphasis, the word ailment is often found modified by the adjective minor.
Let us note that, just like there are adjectives associated with the nouns illness and sickness, there is also an adjective associated with the noun disease, namely diseased /dɪ.'zizd/, which means ‘suffering from a disease’ (CALD). This adjective is derived from the past participle of the now obsolete Middle English verb disesen, borrowed in the 14th century from Old French desaaisier ‘to deprive of ease, make uneasy’. The adjective diseased is first attested in the middle of the 15th century. Whereas the adjectives ill and sick are typically used to refer to diseased individuals, the adjective diseased is rarely used to refer to a person. It is more often used to refer to an animal, as in a diseased cow, or to body parts, as in diseased liver. Also, the adjective diseased has a much stronger sense and it sounds more serious than the adjectives ill and sick do.
Spanish synonyms of enfermedad
Spanish does not have serious (competing) synonyms for the noun enfermedad and we find that this noun is the best way to translate all three: disease, sickness, and illness. However, the synonym ailment does not typically translate as enfermedad, but rather as dolencia, achaque, or even mal. The latter can be used as a synonym of enfermedad in some contexts.
Sp. dolencia ‘ailment, complaint, illness’ used to be a synonym of enfermedad in Old Spanish, and it was even the preferred word. It can still be used as a synonym, though dolencia sounds less serious than enfermedad. This noun is derived from the verb doler ‘to hurt’ and thus it involves the idea of something that hurts. The noun dolencia often entails a prolonged or chronic ailment, unlike the noun dolor ‘pain’.
The noun achaque also means ‘ailment’ and contains the idea that it is a chronic one, albeit not a major or fatal one. It is often found in the phrase achaques de la vejez ‘old-age ailments’. This noun is derived from or related to the Spanish verb achacar ‘to impute, attribute, accuse’, a 13th century borrowing from a variant of Arabic شكا (šakā) ‘to complain’ (note that the verb achacar has nothing to do with health issues). One might think that the noun achaque is just a noun derived from the verb achacar, as many other nouns with the ‑e inflection are derived from the stems of verbs, such as cortar ~ corte ‘cut’, empujar ~ empuje ‘push’, and so on. However, it seems more likely that the noun achaque comes from an Arabic noun related to the Arabic verb we just saw. That is because the Arabic noun meant ‘complaint’, but also ‘body ailment, habitual illness’, which is the meaning that Spanish achaque has. It is not too hard to see how the sense ‘ailment’ might derive from the sense ‘complaint’.
Finally, the Spanish word mal can also be a noun meaning ‘sickness’, in addition to ‘evil’, depending on the context. Thus, in luchar contra el mal ‘to fight against evil’, it has the latter meaning and in tener mal de estómago ‘to have a stomach ache/illness’ or in tener muchos males ‘to have lots of ailments’, it has the former one. Although in these examples, mal is used as a noun, Sp. mal is primarily an adverb that means ‘bad(ly)’ or ‘ill/evil’ (in the adverbial sense, seen above). In certain contexts, however, adverbial mal translates as sick, as for example in encontrarse mal ‘to feel sick’.
 The most lethal diseases are tuberculosis, malaria, and AIDS, which kill around 5 million people a year (out of 300 million cases). Tuberculosis alone causes about 2 million deaths per year. The most common infectious disease is Hepatitis B, which has infected a quarter of the world population, but does not kill you right away (chronic infections can eventually cause cirrhosis of the liver or liver cancer). The flu (influeza) kills 20,000 people in the US each year, though there are occasional flu epidemics that can be quite lethal, such as the so-called Spanish flu, which killed half a million people in the US in 1918. Malaria affects as many as 500 million people each year and kills between one and three million. Measles is still found in the developing world, despite the existence of a vaccine, and it kills as many as a quarter million people a year.
 The English adjective easy /ˈi.zi/ (Sp. fácil) is, of course related, to ease. It comes from Old French aisié (Modern French aisé), past participle of the verb aisier ‘to put at ease, facilitate’. Obviously, originally it meant something like ‘(put) at ease’. Note that the adjective uneasy, derived from easy with the oppositional suffix un‑, does not mean its opposite of easy, but rather the opposite of what easy originally meant, ‘anxious or uncomfortable’ (COED).
 The adjective fĭrmus had a short ĭ in Classical Latin, which explains the e in Spanish enfermo, for Latin short ĭ always became e in Spanish. In some variants of Latin, and more specifically of Vulgar Latin, however, it seems the short ĭ became long ī. This explains the i in Sp. firme, which is not a learned word, as the i might seem to indicate. Because the initial f was maintained, and did not become h, however, we take this word to be semi-learned, since the words firmar (‘to firm up; to sign’ in Modern Spanish) and firme (‘firm, steady, etc.’) were common in learned, juridical language. As to why the final vowel is e, it is because Classical Latin fĭrmus changed to fīrmis in Vulgar Latin and that is the source of Sp. firme. Note, however, that the first attestation of this word was a fermo, the patrimonial descendant of Classical fĭrmus.
 Eng. ill is a loanword from Old Norse. When it was borrowed, around the year 1200, it meant ‘ill, bad, wicked, difficult, injurious, etc.’ and it was treated as a synonym of (unrelated) evil. At first, it meant ‘morally evil; wicked, iniquitous, depraved, vicious, immoral, blameworthy, reprehensible’ (OED) when used as an adjective and ‘wickedly, sinfully, blameworthily’ when used as an adverb. Nowadays, ill is used primarily as an adverb, though it can also be used as an adjective in mostly set phrases such as ill health and ill temper (but not ill person). The sense ‘bad’ can be seen in the latter, as well as in compounds such as ill-advised and when used as a noun, as in social ills and to speak ill.
 Spanish speakers have a hard time distinguishing between the adjectives diseased /dɪ.'zizd/ and deceased /dɪ.ˈsist/ ‘someone who has died, especially recently’ (DOCE). That is because the two words only differ in the sounds [s] ~ [z], which are not distinguished in Spanish (in this phonetic environment).