but it had been 19 years since the last wave, so there was a whole new population set up for the Great Plague of 1666. Quarantine worked, but it was incredibly cruel in the cities, where if one family member fell ill, the whole family was locked in with that person for 40 days, pretty much ensuring the whole family would be infected and wiped out. There were cities on the continent that had actually prepared for plague and had built quarantine hospitals to house the sick and dying, thus saving much of their population from death.
Adding to this was a huge influx of rural people into the cities, driven out by the Enclosure Acts that authorized the land held commonly and used by the poor to graze their lone cow or sheep to be enclosed and owned by the gentry. People who had eked out a living with a combination of that and causal farm labor were left completely landless with no way to support any independent source of food. London was especially overcrowded with "surplus population," even with the policies of exporting people to penal colonies in North America and later, Australia.
Overcrowding, chronic malnutrition, and the long gap between waves of plague combined with a silly policy of exterminating cats and dogs (who ate the rats whose fleas caused the spread) ensured the maximum number of dead. Quarantining whole families just made it much worse. The Great Fire of London has been widely credited with ending the plague as it incinerated the rats and their fleas.
IOW, quarantine can work, but it has to be done wisely. This is a documentary on how it wasn't done wisely:
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