Supposedly there was the case of a British officer named Amherst who tried to give Indians smallpox blankets during the Pontiac War in 1763. There has never been any other evidence put forth of any other troops, American or British, doing something like that.
And for the record, leftist historians generously overestimate the numbers of Indians in N.America at the time of European settlement. Some historians claim there were 15-20 million Indians living north of the Rio Grande at the beginning of European settlement. There are no facts to support that large a number. 2-3 million is more likely.
Did many Indians die from diseases contracted from Europeans? Yes, because they had no immune systems that could counteract the diseases. But European settlers still died in large numbers from diseases as well. That was a risk of normal life for everybody back then. And to be sure, many millions of Europeans died from diseases (Black Death, bubonic plague) that came from other parts of the world. Life was brutish and short back then.
My own experience after reading numerous historical books is that while many historians (even ones identified as liberal) will be truthful in their accounts, a number of them have to be regarded with a grain of salt.
Though not yet quantified, there was a large die-off among the Indigenous Americans, and seemingly before the Whites were recorded in America.
Plagues are plagues.