Mark Liberman (at LanguageLog) has been posting on "Language as a Virus" and now expands (with help from Cosma Shalizi) to "Culture" (specifically, religion) as a contagious phenomenon, citing Pliny (Ep. 10.96.9) on the Christians:
Neque civitates tantum, sed vicos etiam atque agros superstitionis istius contagio pervagata est.
This phraseology is likely derived from Livy's depiction of the spread of the Bacchanalia (39.9):
Huius mali labes ex Etruria Romam veluti contagione morbi penetravit.
["The infection of this mischief, like that from the contagion of disease, spread from Etruria to Rome." (Google Books)]
Cf. R. M. Grant, "Pliny and the Christians," Harvard Theological Review 1948: 273-4 [link via JSTOR].
One more: Tacitus, discussing measures against Egyptian and Jewish rites, (Ann. 2.85) mentions 4000 freedmen "infected with that [sic] superstition" (quattuor milia libertini generis ea superstitione infecta)...although "infecta" by itself is not as specific as the English "infected."
The Christians gave as good as they got, of course: The metaphor appears, for example in the title of Theodoret's great work of apologetic, Graecarum Affectionum Curatio, "Cure of the Greek Maladies" [the Greek title being: Hellenikon therapeutike pathematon]. (Greek text via Google Books)
For later incarnations, note Ludovico Nogarola (16th cen.) on Gian Matteo Giberti's actions against heresy (i.e. Protestantism), presuambly with Livy in mind also:
ille...vir prudens curavit sedulo, ne hoc tam late disseminatum malum Germaniae vicinitate veluti contagione morbi ad nos serperet. [Google Books]
And for more examples in Medieval times, see R. I. Moore, "Heresy as Disease," in The Concept of Heresy in the Middle Ages (11th-13th C.), ed. by W. Lourdaux and D. Verhelst (Leuven, 1976), pp. 1-11.