Plague pit is the informal term used to refer to the mass graves with the mortal remains from victims of epidemics of medical conditions caused by diseases such as the Black Death. Often hundreds of bodies would be buried in a single location, as the risk of further infection from traditional funerary rites was too great. It is unlikely, in the case of the Black Death, that this practice had any appreciable effect on slowing down the spread of the disease.
At the start of the plague outbreak, parishes did the best they could to provide proper burials for their parishoners, but soon ran out of space and began to dig mass graves within the city. However, the plague was so devastating that soon, in late 1665, the group graves began to be dug outside the city.
From:eastlondonhistory.com The Plague & the Thames
The graveyards were filled, and Aldgate was turned into a giant plague pit. Daniel Defoe, author of Robinson Crusoe, wrote: “They dug the great pit in the churchyard of our parish of Aldgate. A terrible pit it was... about 40ft in length, and about 15 or 16ft broad... about 9ft deep, but it was said they dug it about 20ft afterwards. “For though the plague was long a-coming to our parish yet, when it did come, there was no parish in or about London where it raged with such violence as in the two of Aldgate and Whitechapel.”