Good to see us back on topic.
I had been thinking about the possibility of IE people possibly not so much bringing diseases on their persons but arriving with built-in immunities that the original inhabitants did not have. I am not an expert on this part and would welcome some correction, but I wonder if those who lived in Europe prior to the IE arrivals had enough of a pig or cattle economy to have developed an appreciable amount of resistance to animal-to-person transmission.
It is well known that stock rearing was a big part of IE culture. Also well known is that 15th and 16th century ACE Europeans had developed immunities not only to person-to-person communicable diseases, but also to diseases carried by animals common to the farm. I would understand if someone were to argue that any of these immunities could have been acquired in the generations after the IE arrivals, but the possibility that all or some of these immunities may have been developed by IE peoples closer to their homeland and prior to the migrations is intriguing.
If the latter was the case, we have the possibility of sort of an indirect form of bringing in diseases for which the original inhabitants had no immunity but would not otherwise spread across wide areas. Animal-to-person communicated diseases require, for the most part, long-term direct contact between the person and the carrying animal, so it is unlikely that that microbe is going to travel very far if those who work around those animals have any appreciable immunities.
Now, if we take these people and give them some motivation to pack up and leave, and they in turn bring their animals with them, we have the scenario where diseases may have been brought along. In this picture, we have IE people arriving with their stock rearing culture and animals and actually introducing new microbes into the area in which they settle. Pockets of these diseases would be set up all over the continent. They themselves do not generally get sick in this case since stock rearing has been with them for some time but the original inhabitants, not having the benefit of inherited resistance, may in fact get sick, possibly in relatively large numbers. In effect, we would be looking at immune people bringing their disease-carrying vectors along with them. Exposure of the original inhabitants to these vectors could have caused many people to get sick until immunities were acquired by successive generations. This scenario could also explain one reason that IE people became so populous relative to those who lived in Europe prior to the migrations.