Zerna Cassius felix impetigines greci lichenas vocant, latini vero zernas et cetera.
Zerna, according to Cassius Felix, is the same as impetigines, which the Greeks call lichenas, but the Latin-speakers call it zernas, etc.
This is, as indicated by Simon, a direct quote from Cassius Felix’s De medicina 11, 1, ed. Fraisse (2001: 24): Ad impetigines, id est zernas. 1 Impetigines quas Graeci lichenas uocant, Latini uulgo zernas appellant.
Zerna, acc. pl. zernas, appears in a multitude of variants: serna, tzerna, sarna, serna, sernia. It occurs in the writings of many authors, e.g. Dioscorides, Pliny, Celsus, Theodorus Priscianus, Isidore of Seville, etc. It is generally considered to be of Iberic origin.
Impetigo. The Latin word inpetigo/impetigo, acc. pl. impetigines, is derived from impeto "to rush upon, assail, attack". It is glossed by Lewis & Short as "scabby eruption on the skin, impetigo." Impetigo in modern medical terminology is well-defined as a bacterial skin infection caused by straphylococci and less often by streptococci. It is common in babies and children, affecting face and limbs. However in medieval times and in antiquity the term covered a broader spectrum of scabby skin eruptions.
Lichen. The Greek word λειχήν /leikhḗn/, Latinized as lichen, lichenas is the Greek acc. pl., originally means "tree-moss, lichen"; this meaning is then transferred to "lichen-like eruptions on the skin".