The plague Heathcliff is likely referring to is the Black Death, a pandemic that hit Europe in the 1300s, wiping out 30-60% of its population and returning several times until it finally disappeared in the 19th century. The disease is thought originally to have been introduced to Europe from Asia, spreading through fleas carried by black rats. The Black Death occurred in three forms; bubonic, pneumonic and septicemic plague. All three forms were highly contagious and death usually occurred within a week of infection. In order to slow the spread of infection, houses where an occupant had been infected were marked with a red cross on the door and locked from the inside.