Old texts call a widespread, deadly epidemic a 'wenli' (a great plague).
Tips
history
疠 is literary and not used independently in modern Chinese. It survives in bound terms like 瘟疠 (great plague) and 疠风 (leprosy); the everyday words are 瘟疫 (epidemic) and 麻风 (leprosy).
register
Literary and historical only — classical medical and historical writing about plagues, not modern speech.
万 stands inside the sickness frame. In the traditional form 癘 the phonetic was 厲; the simplified form substitutes 万, which can also be read as 'ten thousand sick people' — a whole population struck by plague.
