The disease known as rubella [ɹu.ˈbɛl.ə] in English and rubéola or rubeola in Spanish is ‘a mild, infectious, communicable viral disease, characterized by swollen glands, esp. of the back of the head and neck, and small red spots on the skin’ (WNW). It is caused by the rubella virus of the genus rubivirus or the Togaviridae family. The most dangerous thing about it is that it may result in ‘congenital defects in infants born to mothers infected during the first three months of pregnancy’ (AHD). Other names for this disease are German measles or three-day measles, since it results in a similar rash of red spots in the body.
Rubella is known in Spanish as rubéola, rubeola, or sarampión alemán, the latter being much rarer and obviously a calque of the English name. Note that in English, rubeola is another word for regular (English) measles, not for German measles or rubella, which is a totally different disease.
The word rubella is a late 19th century New Latin scientific name for this disease in English, but not in other languages. It is derived from the neuter plural form of the Latin adjective rŭbellus ‘reddish’ (fem. rŭbella), the diminutive of ruber ‘red’ (rŭb‑ell‑us). It was no doubt created on the pattern of the already existing word rubeola. Besides German measles, another earlier name for the disease was rötheln, a borrowing from German, used in the mid-19th century. In German, Rötheln was a plural diminutive noun derived from the adjective rot (earlier roth) ‘red’ (Röth‑el‑n). The modern spelling of this word in German is Röteln [ˈrøːtl̩n], though Rubella is also used in German. Spanish seems to have borrowed the word rubéola from French rubéole [ʀy.be.ˈɔl], derived in the mid-19th century from Latin rubeus ‘red’ on the model of existing French rougeole and roséole (cf. Port. rubéola).
The Latin adjective rŭber meant ‘red, ruddy’, with an alternative regular form rŭbrus (the adjective’s feminine form was rŭbra, neuter rŭbrum; regular root: rŭbr‑). Thus, the diminutive rŭbellus is not a regular derivation. Lat. rŭber descends from Proto-Indo-European *h₁rudʰrós ‘red’, from the root *h₁rewdʰ‑. Another word derived from the adjective rŭber is the adjective rŭbĕus ‘reddish’ (rŭb‑ĕ‑us), from which comes patrimonial Sp. rubio ‘blond’ (rubia ‘blonde’). Also from the Latin adjective rŭbĕus comes Fr. rouge [ˈʀuʒ], adjective and noun meaning ‘red (color)’. This word has been borrowed into English as rouge [ˈɹuʒ] to refer to ‘a red powder or cream used as a cosmetic for coloring the cheeks or lips’ (COED). Derived from the same rŭbĕus is the Lat. rubeola, the source of the name for rubella in Spanish, rubéola or rubeola, and the source of the other name for measles in English, rubeola (see §34.3.26 above).
The rare Latin adjective rŭssus, which also meant ‘red’ (fem. russa, neut.: russum), is also derived from the Proto-Indo-European root *h₁rewdʰ‑ ‘red’ (*h₁rowdʰ-s-o-). This word is usually said to be the source of Sp. rojo [ˈro.xo], Fr. roux [ˈʀu] (fem. rousse [ˈʀus]), It. rosso [ˈros.so], all basic adjectives meaning ‘red’ today. Actually, Sp. rojo is more likely from the even rarer derived word rŭssĕus ‘reddish; bright red’ or else the Spanish word would be roso, not rojo.
The word rojo is unusual because although it seems like such a common patrimonial word, it is not attested until the 16th century. That is because the normal name for the red color in Old Spanish is bermejo. The alternative words colorado and encarnado appear around the same time as rojo. The reason seems to have been that Sp. rojo was originally the word for ‘reddish’, not for ‘red’, including the red in ‘red’ hair color. Hence the compound pelirrojo/a, an adjective meaning ‘red-haired, ginger-haired’ and a noun meaning ‘redhead’. The reason for bringing the rare word rojo for the basic color may have been influenced by other Romance languages.
 Actually, the French word rubéole was created in the mid-18th century but with a different meaning, namely the name of the madder plant, ‘a southwest Asian perennial plant (Rubia tinctorum) having small yellow flowers, whorled leaves, and a red root’ (AHD).
 This mid-18th century loanword is one of the rare words that ends in the sound [ʒ] in English.