There are oral traditions and histories detailing the impact of disease on the Great Plains. Here is an earlier answer I wrote about timing the arrival of disease spread by examining Winter Counts...
Northern Plains tribes (like the Lakota, Kiowa, Mandan, and Dakota) kept historical records in the form of Winter Counts. Winter Counts were a historical record, a list of year names representing the most significant events in the life of the band. Pictorial representations of that event served as a reminder, a kind of mnemonic device, for the keeper of the count to retell the history of the band. We know of 53 Winter Counts that together provide a historical record of the Northern Plains from 1682 to 1920. By compiling the Winter Counts together into a master narrative we can establish a chronology, cross-check errors, and be fairly certain the events depicted are accurate to roughly two years. From this narrative we can determine the frequency and impact of infectious disease on the Northern Plains populations before the arrival of permanent European-descent settlers.
All but two of the 53 Winter Counts record some instance of infectious disease between 1682 and 1920. If we ignore the earliest Winter Counts (due to lack of cross-reference capacity) and focus on the time period from 1714 to 1919, Native American populations on the northern plains endured 36 major epidemics in two centuries. An epidemic occurred roughly every 5.7 years for the entire population, but varied by band. The Mandan saw the recurrence of epidemics every 9.7 years, while the Yanktonai averaged an epidemic every 15.8 years. The longest epidemic free interval for any band was 45 years for the Southern Lakota, and the shortest was 14 years for the Mandan. Northern Plains pandemics, when an epidemic effects all, or nearly all, of the Northern Plains populations, occurred in 1781 (smallpox), 1801 (smallpox), 1818 (smallpox), 1837-38 (smallpox), 1844 (measles or smallox), and 1888 (measles).
Taken together, we see a picture develop, one where epidemics were raging in at least one portion of the northern plains during the eighteenth and nineteenth centuries. Epidemics seemed to hit pregnant women particularly hard, with increased mortality noted in expectant mothers. Overall mortality for each epidemic is difficult to determine. The Blue Thunder (Yanktonai) Winter Count states many died in the 1801-2 smallpox epidemic, but few died in the 1837-38 or 1844-45 epidemics. Oglala Winter Counts describe the 1844-45 epidemic as severe and widespread. The severity of the mortality from an epidemic likely varied between groups due to previous exposure to the pathogen (leaving the survivors with immunity) as well as nutritional stress since periods of famine often preceded an epidemic event.
What does this tell us about disease events beyond the frontier? Epidemics of infectious disease occurred before significant, sustained face-to-face contact with Europeans (3-5 epidemics before the establishment of permanent trading posts). Epidemics of infectious disease arrived in waves, one roughly every 5 to 10 years, burned through the pool of susceptible hosts, and left long periods of stasis in their wake. An entire generation could be born, live and die between waves of disease for some bands, while others were hit with multiple events in quick succession. Even in the same epidemic of the same pathogen, mortality could differ based on immunity from previous exposure and the stressors (famine, poor nutrition, displacement, etc.) influencing the health of the band. Winter Counts tell a story of dynamic populations persisting and adapting in the face of recurrent high mortality events, and provide an unique perspective into the influence of disease on populations beyond the frontier.
Sundstrom (1997) Smallpox used them up: references to epidemic disease in Northern Plains Winter Counts, 1714-1920. Ethnohistory.
Calloway (2003) One Vast Winter Count: the Native American West before Lewis and Clark.
I can't speak to the modern-day US, but from colonial Mexico we do have written accounts of the plagues that wiped out around 90% of the population by the early 17th c. Adapted from an earlier reply of mine:
The plague then called "cocoliztli" was the first major epidemic after the conquest in large parts of Mexico, between 1545–1550, wiping out somewhere around 80% of the native population there. Another huge plague came in the 1570s in once again killed about half of the remaining native population in many regions. They would only start recovering very slowly during the 17th century.
As to contemporary sources mentioning this epidemic from the 1540s I can think of two native authors:* Domingo Chimalpahin* and Juan Buenaventura Zapata y Mendoza. I'll discuss some Spanish sources for this time-frame in the second part as a comparison - this is by no means a complete run-down, but rather some accounts I could find that shed light on contemporary reactions.
Chimalpahin was a lesser noble from the Chalco province, who lived most of his life in Mexico City. He worked as a copyist for the Convent of San Antonio de Abad under the Capucins, until it was taken over by another order. In parallel to this work he also produced the largest corpus we know of any author in Nahuatl: Mostly historical writings on pre-colonial Chalco, the Mexica and other population groups; but also copies of Spanish authors like Gómara.
Less typically, we also have an annal from him written about his own time when living in Mexico City, later simply called the "Diario" (diary), written sometime in the early/mid-17 th century. The pre-colonial annals genre noted important events by year, focusing mostly on the deeds of rulers and nobles. Here's what Chimalpahin notes in it for 1545, in an unadorned writing style:
And also, it was 64 years ago, in the year 1 House [Nahua date], 1545, that blood came out of people's noses. In this year the San Hipólito market was set up. That is how many years it has been until now at the end of this year of our lord 1608. (Here from the English translation called "Annals of his time", transl. Lockhart/Schroeder/Namala)
On to Zapata y Mendoza: He was a noble from Tlaxcala, who was active in the Indian City council for decades in various positions. The Tlaxcalans had been enemies of the Aztec/Mexica, but had profited from being major allies of the Spaniards, and lobbying ot receive special rights in the early/mid 16th c. However, by the later 16th c. Tlaxcala's special status (e.g. with Spaniards officially not allowed in the region) was becoming gradually undermined, and Spaniards started having more political and economic control.
Zapata y Mendoza reflects on such changes in his Historia cronológica de la Noble Ciudad de Tlaxcala, written between 1662 and 1692, so even later than Chimalpahin. He wrote in Nahuatl and in the annals form as well.
As the noble's focus is on native administrators from the city council, he remarks on how many of them died due to the pestilence, with some only serving parts of their terms. He also notes already in 1544 how there had been a major draught in Tlaxcala. He says for '45 that "in this [year] came the great epidemic" (my transl.). Another native author added later here that "The Pestilence [was] in New Spain.". For '47 Zapata y Mendoza writes of Hernan Cortés' death.
It's interesting to note here that while Chimalpahin does not talk of the next major epidemic from 1576-78, Zapata y Mendoza says that in 1576 "there occured a great epidemic". However I'm not sure if this could mean that this later epidemic was stronger in Tlaxcala than in Mexico City - or if it simply points to the two authors different viewpoints.
I'd also like to highlight regarding the very "matter-of-fact" tone of both passages, that both authors started writing in much more detail in those works only when talking about events that had happened in their own lives. In the end, we don't get any attempts at explanations for the epidemic by the scholars, but rather an idea of the scale and horror of the epidemic. Then again, it might be improbable to expect a (modern) medical explanation from someone lacking a medical education and writing in the 17th century.
For comparison I looked into two other anonymous annals from Puebla and Tlaxcala from the 17th century (translated by Camilla Townsend, as "Here in this year"). Once more, the epidemic of the 1540s is mentioned very briefly here, with the Puebla annal claiming that "800.000 indigenous people died of it in all the realm." Which is of course far below the real numbers of casualties, but tells us something of the estimates at the time.
For Tlaxcala there is once more mentioned a famine in the year 1544, which might be in some way connected to the epidemic Charles Gibson (In "The Aztecs Under Spanish Rule", p. 136) also mentions native Tlaxcalan writers talking about a thousand people dying in Tlaxcala every day at that time. While not very specific, we still get a sense I find of the horror and upheavals the epidemics must have presented for the communities, uprooting families and towns.