Laudator Temporis Acti

Juvenal 2.78-81 (tr. G.G. Ramsay):

This plague has come upon us by infection, and it will spread still further, just as in the fields the scab of one sheep, or the mange of one pig, destroys an entire herd; just as one bunch of grapes takes on its sickly colour from the aspect of its neighbour.

                        dedit hanc contagio labem
et dabit in plures, sicut grex totus in agris
unius scabie cadit et porrigine porci        80
uvaque conspecta livorem ducit ab uva.

Susanna Braund ad loc.:

78 contagiŏ: of various sorts of infection or pollution, e.g. Plin. Ep. 10.96.9 superstitionis istius contagio; cf. Hor. Ep. 1.12.14 inter scabiem tantam et contagia lucri, Luc. 3.322 scelerum contagia. On the prosody see Introduction §8. labem 'stain', cf. Tac. Hist. 3.24 abolere labem prioris ignominiae.

79 grex: a herd of any domesticated animals, here pigs (porci 80).

80 'dies because of the scab and mange of a single pig'. For the combination of scabies and porrigo cf. Lucil. 1115 W = 982 M corruptum scabie et porriginis plenum, of an old lion, and figuratively at Fronto 2 p. 112 (Haines) = 159 (van den Hout) scabies porrigo ex eiusmodi libris concipitur. Both words denote forms of eczema (itchy, blistering skin), cf. J. 8.34-5 canibus pigris scabieque uetusta leuibus. On the contagiousness of scabies see Otto scabies 1.

81 'the bunch of grapes takes on discoloration from the sight of another bunch', conspectā ... ab unā lit. 'from a bunch of grapes having been seen', J.'s (per)version of a proverb, uua uuam uidendo uaria fit (see Otto uua). For uua ... liuorem ducit cf. Virg. Ecl. 9.49 duceret ... uua colorem. liuor denotes a bluish colour generally caused by bruising, cf. J. 16.11 nigram infacie tumidis liuoribus offam and OLD 1, interpreting the present passage as 'a taint'. Although liuidus can be used to describe the normal process of grapes ripening (Hor. Od. 2.5.10 with Nisbet and Hubbard), the emphasis here is on disease.

E. Courtney, "The Interpolations in Juvenal," Bulletin of the Institute of Classical Studies 22 (1975) 147-162 (at 158):

The scholiast on 81 quotes a proverb uva uvam videndo varia fit, corresponding to the Greek βότρυς πρὸς βότρυν πεπαίνεται. This Greek proverb refers to envious emulation, which is not in point here; but this notion apparently suggested to Juvenal the word livor, which often indicates envy, but here refers to the colour of ripening grapes (so Prop. 4.2.13, Hor. Odes 2.5.9-12). Yet it is desirable for grapes to ripen, and I wonder if the line is spurious; Wirz, Philol. 37 (1877) 300 showed some uneasiness.

Courtney refers to Hans Wirz, "Beiträge zur kritik und erklärung des Iuvenalis," Philologus 37 (1877) 293–301.