The word “quarantine” stems from the 40 days that a ship had to remain isolated before docking. In 15th-16th century Venice, that is, during the Black Plague.
Not that there probably was a lot of study, on why exactly 40 days. It is likely that people simply relied on old wisdom or rules from the past.
“It rained 40 days and nights“, “40 days of lent“, “40 days in the desert“, “40 years in the desert“.
In the Bible, numbers are not just numbers. They are recurring according to a similar situation. The number 40 always represents a period of probation, trial and chastisement.
When, say, the number 20 is mentioned, and it is mentioned over a 100 times, it is most often for a perfect waiting time, and the completion there of. For example: for 20 years the children of Israel waited to be freed from the king of Canaan, for 20 years Jacob waited to get possession of his property and to be freed from his father-in-law.
The number 2 always stands for duality. The good thief and the bad thief. The good son and the bad son. The patient woman and the complaining one. It’s the number of ultimate opposition.
Combine these latter examples, with the numerous times that it is mentioned that “for God a 1000 years are like one day“, and you know why many Christians have been mentally preparing for 2020.
For sure, that might all be like reading tea leaves, but hey, as humanity as a whole, religious or very rapidly mainly not, we are all still stuck in these ancient algorithms – still living in 7 days; a system either stemming from Israel, or from ancient Babylon (when the jews where in captivity there), letting the land rest in the 7th year, the 7 years of crisis, and agreeing on their wisdom. So the search to dig for potential other wisdom is only normal.
Of course the one-eyed CNNs of this world will quote the “nobody knows the day or the hour“, – and then skip that we can know the season, as well as Jesus’ emphasis on those who can “read the signs of the times“.
The wisest person is the one who looks through 2 eyes, and sees the dimension in the middle.
There are 340,000 cross references in the Bible: see the visual representation in the header.