Hawaii

However, contact with Western civilization turned out to be disastrous.

An excerpt from the article 16 facts about Hawaii

The newcomers brought with them various diseases that decimated the native population. Around 1770, the population of Hawaii was 300.000 people, in 1850 it dropped to 60.000, and in 1920 it was only 24.000. Smallpox took the greatest toll on the islanders.