There's very less evidence to support that. I can only recall one incident where a blanket infected with small pox was handed out to a tribe and the officer who did that was later severely reprimanded.

But anyhow, I am not trying to downplay the ill intent of European settlers, I will just quote this here "history is written by victors". The blanket incident is all I recall about deliberate disease spreading. who really knows? How many hundreds of such incidents went unreported?