how the sickness started were by the rats because rats have fleas and those fleas went on people which that made the bacteria and got people sick.
They went to wars to fight and they brought rats and people would die 3 to 5 days only because of the fleas when the soldiers others soldiers dead they would through them to the other side of the village.
Since there was a lot of soldiers and lot of people dead they thought that they did something wrong and that god was punishing the people in the village. So everybody had to stay inside and cover every hole.
There was a lot of digusting stuff on the streets during the middle ages. There were lot of dead people, and dogs. they thought staying inside and not going outside would help them keep out from the bacteria.