A man buried in Latvia some 5000 years ago had an early form of Yersinia pestis [the plague] – see https://phys.org/news/2021-06-year-old-earliest-strain-plague.html … and scientists are thinking this might be an early form of the disease which later mutated and became so deadly, being responsible for the Plague of Justinian and the Black Death in the 14th century AD. The report is therefore coloured by mainstream ideas on how diseases develop over a period of time. The suggestion is that because the skeleton was likely that of a hunter-gatherer [farming had not reached Latvia at that time it is claimed] the Yersinia pestis was not as disastrous as in other outbreaks. However, it has long been recognised that there was a sharp fall in population numbers in Europe towards the end of the fourth millennium BC. It also coincides with the Piora Oscillation [see Wikipedia]. Something was going on at this time and the plague outbreak could be just one element, one that has been recognised by modern methodologies.
In the Hoyle and Wickramasinghe book 'Diseases from Space' one might find a parallel, speculative as they might seem. Another period of depopulation is located in the second half of the third millennium BC, allowing migrants from mainland Europe to become a dominant faction in Britain – the so-called Beaker Folk. Genetic research has shown that eastern Europe and what is now Germany, were settled by migrants from the steppes around 3000BC, and these people went on to proliferate in northern Europe and Frisia, migrating into Britain as the Beaker Folk. They may even have brought a proto-English language with them, closely related as it is to Frisian and Germanic variants. They absorbed the surviving Neolithic population who themselves had absorbed the hunter-gatherer population of the Mesolithic era. The effect of major disease, such as the plague, on human history is a little explored subject. We know it played a dramatic role in the Americas prior to and after the European colonisation, but the same must be true of old world history too. See also https://cosmictusk.com