The Collaborative International Dictionary of English v.0.48:

Pesthouse \Pest`house"\, n.
   A house or hospital for persons who are infected with any
   pestilential disease.
   [1913 Webster]

WordNet (r) 3.0 (2006):

pesthouse
    n 1: hospital for persons with infectious diseases (especially
         leprosy) [syn: lazaretto, lazaret, lazarette, lazar
         house, pesthouse]