Lurgi

Lurgi, also known as the dreaded lurgi, is a disease that infects Numberjacks. While humans use the word "lurgi" to mean any disease, the Numberjacks seem to use the word to refer to this specific disease.

Properties
The symptoms of the disease manifest all at once and come on rather suddenly. They also desist in under a day.

The symptoms of the disease include an inability to jump very high, aches all over the body, fatigue, sometimes coughing, and it could also negatively impact the eyesight or the brain as when Four was afflicted, he mistakenly said that there was nobody on the couch. Lurgi also seems to be infectious as they make a reference to "catching" it.

History
In The Dreaded Lurgi, Six came down with lurgi in the Gym. Four was present and had never heard of lurgi, but when Six explained the symptoms, Four got lurgi. When they arrived in the Control Room and were asked by Three what took them so long. Six replied that they had the dreaded lurgi and Three replied that she hoped she didn't catch it, which seems to imply that even though Four did not know what lurgi was previously, she did. After a still-sick Four found a five for Five to land on, Three sent Four and Six to bed. Later, Three got the disease but still managed to use the Brain Gain Machine. She then went to bed. Later that day, all three afflicted Numberjacks recovered and Four was worried that Zero had it, but he was actually fine.

Trivia

 * Seeing as Four didn't know what lurgi was until the episode, this is likely his first time catching lurgi. It is unknown if any of the others have had it before.
 * Lurgi is possibly more serious to even numbers and/or males as Four got it within a minute when in vicinity of Six, while Three only got it much later and Five did not get it at all.