The word scabies ['skeɪ̯.biz] refers to ‘a contagious skin disease caused by a parasitic mite (Sarcoptes scabiei) and characterized by intense itching’ (AHD). The word scabies is also used for ‘a similar disease in animals, especially sheep’ (AHD). The itching is due to an allergic reaction to the mites, which are actually the female mite Sarcoptes scabiei var. hominis, an ectoparasite (Sp. ácaro parásito) that burrows into the skin to live and deposit eggs. Although it is under half a millimeter in size, it is sometimes visible to the naked eye. Mites (Sp. ácaros) belong to the same phylogenetic class (Arachnida) as spiders and ticks. They are ‘any of various small or minute arachnids of the order Acarina that are often parasitic on animals and plants, infest stored food products, and in some species transmit disease’ (AHD).
This disease is sometimes also referred to in English as the seven-year itch, presumably because its symptoms decline after that many years. However, the term is rather imprecise in this regard since it is also used to refer to STD’s, whose outbursts also tend to decline after seven years.
The scabies skin disease was first described and named by Roman medical writer Aulus Cornelius Celsus around 2,000 years ago. The Latin word chosen to name the disease was scăbĭēs, which meant ‘roughness, scurf’ and ‘scab, mange, itch’. This noun is related to the adjective scăber (fem. scăbra, neut. scăbrum, regular root scăbr‑), meaning ‘rough, scurfy, scabrous (esp. from uncleanness)’ (L&S) and/or from the verb scăbĕre ‘to scratch, scrape’. The choice of the word scăbĭes would seem to have to do with the rough aspect of the infected skin and/or the associated itch. English borrowed the word scabies for this disease from Latin around the year 1400.
Spanish did not keep the Latin word scăbĭēs for this disease but, rather, it uses the word sarna, a word common to the Iberian Romance languages and found in Late Latin. Its origin is uncertain but it is thought to be of pre-Roman Iberian origin. It has been suggested that this word is related to Basque sarra ‘rust, iron residue, etc.’, for example, and to words in neighboring Romance languages with various meanings.
Derived from sarna, Spanish has the adjective sarnoso/a which, when speaking of skin, means ‘itchy’ or ‘scabby’, when speaking of a person means ‘suffering from scabies, scabious’, and when speaking of a dog means ‘mangy’, i.e. ‘suffering from mange’, which is ‘a skin disease of mammals caused by parasitic mites and occasionally communicable to humans, typically causing severe itching and hair loss’ (COED). The name mange [ˈmeɪ̯nʤ] for this animal disease comes from Old French manjue or mangeue ‘itch; eating’, which is derived from the verb mangier ‘to eat’ (Modern French manger [mɑ̃.ˈʒe]), from Latin manducāre ‘to chew, masticate’.
Spanish does have a learned, technical word for scabies that comes from the Latin root scabi‑, namely escabiosis, derived from the Latin noun plus the suffix ‑osis that indicates disease in Medical Latin. English does not have an exact cognate for this New Latin term. Spanish also has the medical term is the adjective escabioso, which is equivalent to the popular sarnoso. It is a loanword from the Latin adjective scabiōsus ‘rough, wrinkled, etc.’. English does have a cognate of this adjective, namely scabious, though it is quite rare. It means ‘of or relating to scabies’, ‘having scabs’ (AHD), ‘affected with mange; scabby’ (COED).
The English word scab [ˈskæb] that we just saw in the definition of scabious is an interesting one, for contrary to what one might have thought, it is not related to scabies. The word is a mid-13th century loanword from Old Norse skabb and its main meaning is ‘a dry, rough protective crust that forms over a cut or wound during healing’ (COED). However, presumably because of its similarity to the word scabies, the word scab has come to have the meaning ‘mange or a similar skin disease in animals’ as well as ‘any of a number of fungal diseases of plants in which rough patches develop, especially on apples and potatoes’ (COED).
There is one other Latin word derived from the same root as scăbĭēs, namely the Late Latin adjective scăbrōsus, derived from the also adjective scăber (see above): scăbr‑ōs‑us. English and Spanish borrowed this word in the second half of the 16th century, English as scabrous and Spanish as escabroso/a. Since it is first attested in French in 1501, Fr. scabreux, it is quite likely that both English and Spanish borrowed it through French, not directly from Latin. When said of terrain, Sp. escabroso/a, which means ‘rough, rugged’, but it can also be used figuratively, translating as thorny, tricky, tough, difficult, or distasteful when speaking of a topic or a subject, or as shocking, gory, or lurid when speaking of a scene or a story. The main meaning of Eng. scabrous is ‘rough and covered with, or as if with, scabs’ but it also has a figurative meaning, namely ‘salacious or sordid’. As we can see, the two cognates are false friends, except perhaps in some figurative sense, one which most likely was borrowed through French.
Finally we should mention a well-known Spanish saying, used particularly in Spain, that contains the word sarna, namely Sarna con gusto no pica. Literally it means ‘Scabies that gives you pleasure is not itchy’. The figurative meaning of this saying is that if you enjoy or derive pleasure from something very much, such as scratching an itch, then that more than makes up for the itch. Or, in other words, the benefits of doing something outweigh the negative aspects associated with it. This saying is used particularly to refer to those who pursue some illicit pleasure and who put up with the negative consequences associated with it. A commonly heard retort to this saying is …pero mortifica ‘but it does torment’, which emphasizes that there are still negative consequences to be had for the pleasure-seeking behavior.
 The term seven-year itch was later appropriated to describe the claim that divorces often happen after seven years of marriage. The term was made popular through the play The Seven Year Itch by George Axelrod (1952) and, in particular, its film adaptation starring Marilyn Monroe and Tom Ewell (1955). The movie’s title was translated into Spanish as La comezón del séptimo año in Mexico and Argentina, a fairly literal translation of the English title, but as La tentación vive arriba ‘Temptation lives upstairs’ in Spain.
 Sp. escabiosa is also the name of a plant, also known as lengua de vaca or viuda silvestre, a flowering plant of the Caprifoliaceae (honeysuckle) family, genus Knautia. The scientific name is Knautia arvensis. It is commonly known in English as field scabious. The reason for the plant’s name is that it was used to treat scabies and other skin afflictions.