I've heard that a few times, but it never made sense to me why that didn't go in the other direction - if the Europeans had diseases the native americans didn't

I've heard that a few times, but it never made sense to me why that didn't go in the other direction - if the Europeans had diseases the native americans didn't have immunity to shouldn't the native americans have diseases the europeans didn't have immunity to as well? Seems like that should have just balanced out.