Ummmm , you need to be very thankful that you have never had 'real flu' it is a terrible disease. Did you know that 'flu' after WW1 in 1918 killed more people, around 4 million, than died due to the entire war?
I sometimes work at the company that makes the 'flu ' vaccine 'Tamiflu' every year it has to be reconfigured to take in the new variants of flu that have been introduced. Flu virus mutates every few months to a new variant, some are more virulent than others but all need to be identified and added to the vaccine. Flu is by far the most deadly human disease with the highest death rate that has ever been identified.
Problem is most people think 'flu' is just another name for a feverish cold! but it is far more than that. I had it once, temperature of over 42 C delirious and unaware of where I was or what was happening for 2 days!