The keto flu is almost a necessary evil, to enter the gates of ketosis. The keto flu is your body's natural way of reacting to the lack of carbs. Some ketoers have even described the keto flu to be similar to withdrawing from other addictive substances (e.g. tobacco).
Like most people, you probably have to feed your body a significant amount of carbs most of your life. Our bodies and brain are perfectly happy with carbs and have adapted to through hormonal and metabolic processes, which normally run in the background.
Switching fuel sources will likely be a drastic shock for your body, at first. In a nutshell, keto flu is a name for a set of symptoms, what you will experience, on the way to ketosis....