I often attend and host talks by researchers working on space exploration, and one finding that's been interesting is immune dysregulation in isolation posing risks to astronauts and explorers. Here's a review on the topic, https://www.frontiersin.org/journals/immunology/articles/10....

This type of dysregulation also exists in AIDs. For the edge case of the islanders, then the level of risk and lethality our germs pose for isolated islanders with modern medical care is somewhat disputed, but it would be deeply unethical to ever test. We just don't know. So I won't pretend to know here.

However, these are edge cases that don't matter, because bioweapons, i.e. things deliberately designed to kill, just aren't the same as a dozen isolated islanders, astronauts on a mission to mars, or someone who is severely immunocompromised.

Weapons are designed to kill healthy people. The doomsdays being proposed are super-bioweapons that kill billions of humans. I think it's safe to say that as a rule of thumb, by and large, anything that lethal is lethal enough to kill you.