By refusing to follow the rules set by the medical authorities. Refusing to wear mask, not isolating when infected, etc...

A more extreme example of false beliefs causing great harm is the Holocaust that was caused by Nazis believing the Jews are part of a conspiracy to undermine Germany and were the cause of Germany's defeat in WW1.

False beliefs can cause a lot of harm to a lot of people in both personal level (not vaccinating your kids) and to the larger societal level.