Quarantining is based on 700 year old virus-fighting logic. Here's how to safely shorten your time in isolation, according to science.

• Quarantining is imprecise, 700-year-old virus-fighting logic that originated with the plague in Europe.
• The coastal cities of Dubrovnik and Venice invented some of the first quarantines in the 1300s and 1400s, which were as long as 40 days (that's 'quaranta giorni' in Italian).
• There was no real reason that the quarantines were so long, other than 40 days was more than enough time for fleas to die, and 40 was an easy number for people to remember.
• Modern quarantines for the coronavirus, at 14 days, are also likely a few days longer than they need to be, in the overwhelming majority of cases.
• 10 days of isolation is usually enough to determine if someone is going to get sick with COVID-19.

700 years ago, Venetians were terrified of the Black Death.

The bubonic plague had come to Europe, where it was mostly passed around through flea bites. The disease would quickly make its bulbous presence known, with swollen lymph nodes appearing as large as chicken eggs in people's armpits and groins. Then, the plague would kill its victims - typically, within just days.

In order to fight this pest, the people of Venice, and their neighbors across the Adriatic Sea in Croatia, did something new: they asked ships arriving from infected ports to sit at anchor for quaranta giorni (40 days), or sometimes, just a trentino (30).

The quaranta moniker stuck around and gave us the modern quarantine.

Medieval quarantines in the 13- and 1400s. were longer than they needed to be, but they also worked well. At times, as many as 500 people were dying every day offshore in Venice from the plague, but, they weren't bringing any plague-carrying fleas ashore in trade staples like cloth, to infect entirely new communities.

Quarantines, when conducted properly, have the ability to stop many of the worst infectious diseases in their tracks.

Historians and public health experts generally agree that there was no real logic to why 40 days was chosen, other than perhaps that it was an easy-to-remember time period for medieval Christians. The bible said Jesus fasted for 40 days as he traveled across the desert, and God rained down on Noah's Ark for 40 days too. Why not try 40 days for plague isolation?

The Black Death still arrived in Italy and Croatia via other routes, but quarantines no doubt helped slow the spread of the plague into new areas.

With time, and a better understanding of how diseases transmit, modern quarantines have shorted.

But quarantining is still an imprecise science, and one that typically prizes caution over practicality to ensure that even cases which take an unusually long time to present are discovered - before it's too late.

Just like 40 days, 14 is a 'pretend' quarantine number

When the novel coronavirus arrived in late 2019, the world had a new plague-like disease situation to wrestle with, this time with the addition of fast-moving airplanes.

Early in 2020, disease experts around the world settled on 14 days for coronavirus quarantines, because in the vast majority of cases, that is more than enough time for people to start to get sick.

By May, researchers had found that 97.5% of people who get COVID-19, the disease caused by the coronavirus, do so within 12 days of exposure. But, the average incubation period for COVID-19 is fewer than six days.

Dr. Rishi Desai, who used to teach people how to quarantine for the CDC, said that there was never any real precision to the 14-day coronavirus quarantine rules.

"Fourteen is a pretend number, right? Like, why not 13?" Desai, the CEO of Osmosis, recently told Insider. "It's because it rounds to two weeks, and people can remember two weeks. So, there's no magical reason why 14 days was chosen other than it's just easy to remember."

Like the 40 day quarantines of old (and pretty much every other quarantining guideline that's been used in between) we're now learning that 14 day quarantines for the coronavirus may be a bit of overkill, at least for the vast majority of cases.

Astronauts quarantined for 21 days when they first returned from the moon in 1969, even though no one had any idea how long any lunar contagions might last.

Most smallpox cases surfaced in an average of 12 days from exposure, but smallpox quarantines dragged on for as long as 16 days, with recommendations stretching to 18 or more before that virus was eradicated in 1980.

When SARS landed in Canada in 2003, the country urged anyone who'd been exposed to it to quarantine themselves for 10 days, even though the average time to infection was less than half that long, at 4 to 5 days.

Coronavirus quarantines can be even shorter with tests in the mix

Knowing all this, it's easy to understand why the US Centers for Disease Control and Prevention recently revamped its own quarantining guidance, saying that 10 days is probably enough time to catch almost every infection that might be passed along by someone who was previously exposed to COVID-19.

The coronavirus is arguably a prime candidate for shorter quarantining too, because it appears this virus gets most infectious just as people are starting to feel sick, before they ever show symptoms at all.

"Ten days is where that risk got into a sweet spot we liked, at about 1%," the CDC's COVID-19 incident manager, Dr. Henry Walke, told reporters last week. "That's a very acceptable risk, I think, for many people."

(Walke said many quarantiners weren't spending a full 14 days away from unexposed people anyway, quitting quarantines early to return to school and to work.)

COVID-19 testing, a piece of modern technology not available to Medieval Venetian plague-fighters, can also dramatically change the game. Though lab tests aren't perfect at picking up coronavirus infections, they do pretty well, especially when people are excreting lots of virus. That's probably when patients are most likely to get others sick.

So, in updated guidance the CDC also says that a coronavirus quarantine can now be exited in relative safety, with only 5% risk, after just seven days of quarantine, and a negative test result conducted in the 48 hours beforehand.

Sure beats those biblically long wait times of 40 days on ships.