it's important to rest especially after a fever but it doesn't mean that you can just play immediately after the fact that you just got sick, it's just not recommended
Performing terribily in chess after fever
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Get well soon
Get well soon 
Recently, I had a mild fever for two days but today, on the third day I felt quite normal and well but I am still not performing well in chess. I am facing the following things
•I make obvious blunders which I didn't made before all this, at this level
•I was mostly used to see by intuition, many of them I need to consciously consider
•When solving puzzles, I need to check the solution of more puzzles than before.
•I am more likely to make lazy moves, and its more difficult to maintain focus and not play impulsive moves
What can I do?