The phrase "The dreaded lurgi" (also spelled lurghi, or lurgy) has its roots in the British radio comedy, The Goon Show, a once weekly conglomeration of nonsense perpetrated over many years by Spike Milligan, Peter Sellers, Harry Secombe and, for a period, Michael Bentine.
Used in a number of episodes of the show, it was descriptive of a non-specific illness, laying victims low by the thousand.
Probably because of the resonance and onomatopoeic nature of the word "lurgi" the phrase invaded the consciousness of the public, not only in the UK, but in many of the countries of the Commonwealth, so that it became virtually synonymous with suffering from a non-serious but misery-making affliction, such as the common cold, a stomach upset or whatever. Somewhat equivalent to the Australian phrase "I'm crook", you would be unlikely to take a case of the dreaded lurghi to the doctor, but you might well call in to work sick with it, and would certainly use it as an excuse for not having met social obligations.
I've always found it a delightful phrase, intimating a level of triviality to one's suffering, without undermining the genuine unpleasantness of being ill - it's perfectly suited to the understated, stiff-upper-lip nature of the British psyche, which traditionally describes anything less than a mortal wound as "Just a scratch".