When the phrase was transported to America, it morphed into "sick as a dog," not referring to someone with a cold or flu, but the nauseous reaction one gets after eating something unagreeable to the stomach.
A "clean bill of health" is an indication that someone is cleared to do something because they do not have any serious ailments, as shown by their doctor.
Originally, the bill was an actual document not just for one person, but for an entire passenger crew on a ship.
"A doctor would come on board" and inspect the entire ship for any signs of communicable diseases such as cholera, the Bubonic plague or typhus -- transmittable diseases that would kill lots of people quickly, according to Michigan's Markel.
If there was no sign of any of these diseases, the doctor would issue a document showing that the port the ship sailed from did not endure any epidemic or infection at the time of departure.
This physical medical document has developed into a metaphorical one in language. We now use the phrase to mean that a person is completely healthy and void of any sickness.
We've heard of "keeping the enemy at bay." Well, a variation of that expression is used to refer to the cold and flu.
The phrase had its beginnings in military wars and battles. During the Greek and Roman era, the Bay tree was believed to have unique powers of protection.
Because the Bay tree was seen as a shield of safety, Greek and Roman soldiers would gather under the tree whenever stormy weather struck.
"Roman and Greek soldiers would always shelter under [the tree] during storms," Jack said.
The Bay tree then became a symbol of strength and protection, and became incorporated into the military uniforms to guard them from opposing armies.
"Soon, the men would be wearing them in the belief they would keep the enemy at bay," Jack reported.
When evolved into "keeping a cold at bay," the cold becomes equated with enemy at battle, providing an interesting metaphor for fighting the cold and flu.
You barely can get your eyes to open in the early hours of the morning, yet you need to get up for work. Such an experience is sometimes defined as "feeling groggy."
The British, however, use the term to refer to someone as being sick or ill.
The origins of "feeling groggy" can be traced back to a military general during the 18th century -- Admiral Vernon, "commander-in-chief of the British forces in the West Indies in 1740," Jack pointed out.
"He was known throughout the Fleet as Old Grog, thanks to his trademark Grogam coat," an tidbit Jack related.
Vernon made a strong effort to restrict and reduce rations of rum and alcohol among his fleet of men, an unpopular decision that dissatisfied many. This unpopular ration amount soon became known as Grog.
The term over time transformed into "groggy," which was used to describe any intoxicated and sick sailor. Because of the similarities of symptoms between one who's drunk and someone sick, "feeling groggy" became a term to also refer to illness.
If you get a vaccine for the flu, "you have a cow to thank, both medically and linguistically," according to McFedries.
In 1796, Edward Jenner, a physician, observed that for some reason, milkmaids seemed to be immune from the smallpox infection that was plaguing the population. He saw, though, they got a milder disease -- namely, cowpox.