Herd immunity is easier when 1/3 of the herd died from the disease. But they also did figure out quarantines eventually. "Quarantine" comes from the 40 day wait

Herd immunity is easier when 1/3 of the herd died from the disease. But they also did figure out quarantines eventually. "Quarantine" comes from the 40 day waiting period imposed on ships as a result of the black plague.