Found: Fiction: Girl sent to island for being sick

A girl is accused of being sick and sent to an island of sick people. She drank tea before being tested. They took a boat to the island. Most likely made in the last 10 years.

If it's the book I'm thinking of, it was a Scholastic book about 2015-17. Title of the book was the name of the disease, something like "The Phage" but that doesn't seem to be correct. Good luck.

It's been a while since I read it but could it be The Death House by Sarah Pinborough?

That has children who are found to have a defective gene sent somewhere (and the sea does feature but I can't remember if it is an island or just very remote coastline) and so every child there has this gene and is watched until they become sick.

I think much was made of the fact that one child was almost 16 and if they had made it to their 16th birthday they wouldn't have been tested or sent to the place. Other children had the defect show up at a much younger age.

It was a Scholastic book around 2015-2017

I found it! Thanks to the comments I remembered its name. The Scourge by Jennifer A. Nielsen

Touchstone: The Scourge