Paraguay during the War of the Triple Alliance. there is much about this conflict between Paraguay, Brazil, uruguay and argentina in " Weep, Grey Bird, Weep: The Paraguayan War 1864-1870" by Roger Kohn. estimates of mortality of Paraguay in this conflict is as high as 65% of the total population of Paraguay, perhaps 90% of the male population.

Yes, one example is the collapse of Indigenous communities across North America, Central America, Australia, and Africa, with the arrival of European colonists. This was probably one the biggest depopulation events in human history, after the Black Death. The traditional explanation is the idea of "ecological imperialism" meaning, Europeans inadvertently introduce diseases that Indigenous peoples had no previous exposure to, meaning they were highly vulnerable without any immunity. This idea is controversial on some levels, because it advances the idea that Indigenous peoples were immunologically vulnerable, or weaker than Europeans. So it's like saying, they were bound to die anyway (a concept known as ecological determinism), along the lines of Darwinism and natural selection.

 

It's hard to get exact numbers for these events, because they occurred hundreds of years ago. Scholars can only estimate. It's well-established that a majority of Indigenous North America was wiped out by European pathogens though. No one is arguing that. What's interesting about the collapse of these populations was that there were multiple factors working against Indigenous groups. The socio-economic effects of such a huge loss of people, not just disease alone was most likely the main culprit. Basically, the mass death that occurred had a snowball effect on the collapse of Indigenous societies.

 

So it's not just biology doing the work. Disease environments in the Americas may have been different, meaning Indigenous peoples probably never got smallpox until Europeans showed up, but Indigenous populations of the Americas suffered from the same leading causes of mortality as Eurasian peoples. Acute respiratory and gastrointestinal disease along with famine and violence, were leading causes of mortally in the pre-Columbian Americas, just as they were in the Europe. It usually took two or three generations after the outbreak of intense epidemics (in Europe) for European populations to recover. In the Americas though, high mortality rates through non-endemic disease, combined with military conflicts with colonizers, famines, and mass migration, led to social crisis, societal collapse in some communities, and prevented the recovery of populations. So the cultural impacts of colonialism and the presence of settlers worsened the situation.

 

We see this happening in Africa as well. By relying on data from modern Third-World Indigenous societies, scholars maintain that a lower immunity to diseases such as smallpox played a minor role in their depopulation of African peoples, and that environmental factors were more significant. Example, environmental transformations catalyzed by European settlement near Khoi societies played a larger role in their disappearance. The encroachment of Dutch settlements on Khoi pastureland and the subsequent loss of Khoi livestock as a result of droughts and livestock epidemics (which concurred with a smallpox epidemic in 1713) initiated a process of demoralization of Khoi society. The simultaneous introduction of European disease, combined with the loss of their cattle and pastureland led to the disappearance of Khoi society. For the Khoi, cattle were more than a major source of food – cattle were culturally significant and were symbols of wealth and social standing. With the disappearance of their cattle, the Khoi began to rely on nearby European settlements for food and thus became dependent on Europeans for their survival.

 

Similar events played out in Australia. Think of the Americas and Australia as "ecological islands." So when we introduce alien diseases, animals, plants, and human to these places, there are massive consequences on societies and the environment. Some are advantageous for the settlers, and sometimes it proves to be devastating to local populations.