The European colonists did not "give" the Natives diseases in the sense you seem to be implying. These diseases were beyond the understanding of the people of the time and they did not have the proper knowledge and treatments to prevent or cure such ailments. The Europeans had already built immunities to them but the Natives had not so it took a deadly toll. It is regrettable that it happened but there was nothing that could be done once the ships hit the shores, and it was not intentional on the colonists' part anymore than it was when the bubonic plague came to Europe from Asia a few centuries earlier.
And it is still not relevant to the feast at Plymouth colony all the same. It is only retroactively made so due to the bitter conflicts that took place later.
Now I'm talking about the historical basis for Thanksgiving here. What the holiday means to me is just a day for the family to get together, give thanks to the people and things in life that matter, and have a damn good meal. I suspect that's what it means to most people who aren't hellbent on spoiling American traditions as seems to be a popular activity these days. But I will stick to it no matter how unpopular it might become to do so.
Are we seriously still going over this?
This thread is irreverent anyway.
I did tell you not to go there. But you did.