What is explained in the article Shutting Down Hawai‘i: A Historical Perspective on Epidemics in the Islands

Because one thing Hawaiians know about is epidemics. Foreign diseases have come through here before, and they have inflicted unfathomable damage. Hence many locals have been pushing the mayors and Governor David Ige to shut down the islands completely to outside travel. (On Saturday, Ige ordered that all incoming travelers be quarantined for 14 days and an emergency, statewide stay-at-home order was effective as of this morning.) This is not an easy call, as the visitor industry is a major portion of the economy.

To understand the eagerness behind Hawai‘i residents to shut down the islands to travel, the current epidemic must be understood in geographic and historical context. The Hawaiian islands have been referred to as “the last landfall”: about 2,500 miles from the nearest other island, and further than that from the nearest continent, the islands evolved in relative isolation. Plants and birds that got here adapted to suit the local environment, creating a place where 97 percent of all native plant species and most of the native birds are found nowhere else on earth. The Hawaiian people, arriving here more than a thousand years ago after millennia of migration out of Southeast Asia, were similarly cut off from the rest of their species, and—like the native peoples of the Americas—never experienced the diseases that had affected the Old World. This made them “virgin populations” who had not, through exposure, developed resilience or immunities.

Explanation:

Thus the introduction of the first outside diseases in 1778, with the arrival of Captain Cook, was catastrophic. Cook visited the island of Ni‘ihau, on the far northwestern end of the chain, in January 17 of that year. His journals remark on the health of the people, and the absence of disease. He knew his men were carrying venereal diseases, and he tried to keep them away from the native women. But when their ships were blown offshore, men that were left on the island had to stay for three days. Nine months later when Cook returned to the islands, he found that the venereal disease had spread throughout the entire archipelago. While it’s uncertain exactly which disease it was, the impact was unmistakable. By the time French explorer La Pérouse arrived in the 1790s, he said of Hawaiian women that “their dress permitted us to observe, in most of them, traces of the ravages occasioned by the venereal disease.” The disease did not necessarily kill outright, but it could render the people infertile, beginning the steep downward decline of the Hawaiian population.