
1) Play by Sidney Howard 2) Quarantine flag
Found on
https://www.crosswordclues.com/clue/yellow-jack

A yellow flag flown to show there was sickness or plague aboard.
Found on
http://black-bart.co.uk/html/pirate_glossary.html

The plot line follows the events of the well-known `Walter Reed Boards,` in which Major Walter Reed of the U.S. Army worked to diagnose and treat yellow fever (called `yellow jack”) in Cuba in 1898-1900. The U.S. Army Medical Corps doctors studied the theory by the Cuban doctor Carlos Finlay that the disease was caused by bites of infected A...
Found on
http://en.wikipedia.org/wiki/Yellow_Jack

The yellow jack, Carangoides bartholomaei (also known as coolihoo), is a species of offshore marine fish in the jack family, Carangidae. It is one of only two representatives of its genus present in the Atlantic Ocean, inhabiting waters off the east coast of the Americas from Massachusetts in the north to Brazil in the south, as well as a number o...
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http://en.wikipedia.org/wiki/Yellow_jack

(from the article `jack`) ...attaining lengths of 1.8 m (6 feet). The genus Caranx includes several species of smaller but popular game fish, such as the crevalle jack (C. ...
Found on
http://www.britannica.com/eb/a-z/y/6

Yellow fever
Found on
http://www.encyclo.co.uk/visitor-contributions.php

Yellow jack: 1) The flag displayed from lazarettos, naval hospitals, and vessels in quarantine. 2) Synonym for yellow fever as, for example, in '...Fernando passed a few weeks recovering from a touch of yellow jack.'
Found on
http://www.medterms.com/script/main/art.asp?articlekey=18088

Yellow Jack is slang for yellow fever.
Found on
http://www.probertencyclopaedia.com/browse/ZY.HTM

[
n] - caused by a virus transmitted by a mosquito 2. [n] - yellow flag hoist on a ship in quarantine 3. [n] - fish of western Atlantic and Gulf of Mexico
Found on
http://www.webdictionary.co.uk/definition.php?query=yellow%20jack
[Treasure Island] yellow fever, an acute, infectious tropical disease caused by a virus and spread by mosquitoes.
Found on
https://www.cliffsnotes.com/literature/t/treasure-island/study-help/full-gl
Caranx bartholomaei noun fish of western Atlantic and Gulf of Mexico
Found on
https://www.encyclo.co.uk/local/20974

A yellow flag flown to indicate the presence of an illness, often yellow fever, aboard a ship. Often the flag is used to trick pirates into avoiding potential targets.
Found on
https://www.encyclo.co.uk/local/23412

any carangoid fish, esp. a Caribbean food fish, Caranx bartholomaei.
Found on
https://www.infoplease.com/dictionary/yellow-jack

When a ship flies a yellow flag, it indicates the presence of an ill crew member, such as yellow fever. But this was also a trick that smart pirates used to avoid becoming the target of another ship.
Found on
https://www.piratevoyages.com/pirate-lingo/
No exact match found.