Tonga is so worried about Covid because outsiders brought Spanish flu, measles, and dysentery in 18 century, then destabilized the small country.
So aid workers must deliver relief suplies without contact with the locals.

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Very good! This is a little more natural:

“Tonga is so worried about Covid because outsides brought the Spanish Flu, Measles and Dysentery in the 18th century, which destabilized the small country. As a result, aid workers must deliver relief supplies without contact with the locals.