Answer:
they had an unusual genetic mutation that prevented them from becoming infected
Step-by-step explanation:
In 1665-1666, in an English village called Eyem, 260 people died from bubonic plague. The plague had entered the village in a bale of cloth that had come from London. Only 86 people survived and it is said that the reason why they survived even though they had been in contact with infected people was that they had a genetic mutation known as "Delta 32". This gene was found in the descendants of the survivors.