I think 90% could very well be accurate. The Black Death in Europe was only a few generations passed when Columbus sailed.
I was trying to point out just how virulent Europe and Asia had become. Massive populations in giant cities, animals living in most homes all winter. In most of Eurasia, cleanliness a forgotten thing (compared to Romans and their love of bathing and the Chinese wealthy who still did), the spread of rapid transportation in ships, often laden with disease carrying vermin... It’s no wonder people were sick all the time.
I would say that most of these killer diseases really became widespread right around 200 years or so BEFORE during and after Columbus, thus giving some immunity to those who lived through them.
When the Europeans arrived, all dirty and in rat infested ships, it’s no wonder so many people, living in a much less—by a huge factor—virulent world, succumbed like they did.
Think about it. It just wasn’t one disease, but dozens—all at once. Smallpox, Bubonic Plague, Yellow Fever, Mumps, Measles, influenza (derived from pig, duck and chicken diseases totally foreign to Indians), rinovirus, cholera, pertussis, Typhoid fever, and tuberculosis, just to name a few.
That is HUGE! And not a soul in the New World had ever had any of them. And they came aboard with every new vessel.
I live in Alabama, right on the route De Soto took when he came through and fought Tuscaloosa of the Alibamios. Almost all of De Soto’s men died of disease. Imagine what happened to all the non-immune Indians he encountered. In fact, he named off splendid royal town after royal town along his journey here, each with a temple on a mound and surrounded by extensive agriculture. This was the heyday of the Mississippian culture. They were all gone in 50 years. The Creeks and Cherokees were the remnants of that culture, and the Southeast was almost back to wilderness by the time the French and English arrived.
The Creek town, Tuckabatchee, only 10 miles from me, had a population of 10,000 souls in 1812 when Jackson defeated the Red Sticks at Horseshoe Bend, and that was considered the largest Indian town in all the Southeast.
De Soto encountered dozens of towns this size in his journey from 1539-1542. By 1600, they were all gone.
First: yes, you pointed out the critical reason for why the death toll was so high among New World inhabitants, namely the cumulative effect of all those diseases hitting them at once. Each of these diseases that swept the Old World over the millennia may have had fatality rates of under 50%, but each new big plague usually arrived many generations after the previous one. So the big hits to the population were spread out over many centuries.
Secondly: For times & places that had no reliable census-taking methods, we have to rely on a few contemporaries who recorded their own personal estimates of the death toll. But these ancient correspondents were often making wild guesses, and making crucial assumptions that they didn't realize they were making.
For instance, seeing a completely empty village might seem concrete proof of a 100% death rate. But as some historians have pointed out, depending on the complexity of a society, a certain minimum number of residents remaining in a village are necessary for it to continue to function as a village. Once the number falls too low, anyone semi intelligent would see the writing on the wall - that the village was doomed - and take their family and themselves to greener pastures.
Or say only 5-10% of a village dies, but the toll includes the only blacksmith, the only doctor, and the only tanner. Again, the village is eventually doomed, so move out now. Or what about the case where a village hits 30% mortality, and the other 70% believed the village was cursed or the crops from the entire region were cursed or poisoned or whatever, and moved out.
Bottom Line: It's hard to estimate how much of a sudden depopulation is due to death, vs how much is due to panic migration.