It made people feel very ill and it even killed many people!
It spread around China fast since it was an "airborne" sickness (which means that anyone infected in an area after breathing the sickness would stay in the air for a while)
From China it then spread to other places and in Europe from trade. It made people very very sick.
It was so deadly that it killed 1/3 to 1/2 of Europe's population wiping out cities. SOme cities had hardly an people left of no people left at all because they all died from the black death.