Illness and weight

Mostly fluid loss.

Certainly not fat loss assuming that the typical person in flu symptoms are in a resting state and not a fat burning state.

Muscle loss doubtful. A person recovering from a serious illness is in a state of detraining, but the amount of loss of lean mass probably couldn't even be measured.

It may appear that there is a reduction in lean mass, but there is a good amount of fluid contained in the muscle cell and a holistic loss of body fluid will temporarily make your muscle mass appear and feel flat until recovering to a healthy state as training and nutrition returns to its normal schedule.

We sometimes hear a phrase "muscle memory". In my small world of discussions that phrase refers to experienced athletes that either from illness, injury recovery, scheduled time off from training go through this detraining state. Those individuals return to condition much faster than those who are just starting to train for the first time in their life and have a tall ladder to climb. In a primitive sense my muscle may return to a highly trained state more quickly after a layoff than a newbie who has the same layoff.

In summary you will be okay to let the illness run its course while you rest and not return to training until you have recovered.

Cheers mate.
I'll continue resting and stop worrying 🙂