A lot of it will come back, because you didn't really lose fat, or weight. You just probably expelled significant amounts of water/liquid during the flu, thus dehydrating you. Even with an increase in fluids yourself, the flu has that tendency to dehydrate us pretty easily.
I'd bet you neither "lost" nor will "gain" much weight because of the flu because it was probably mostly water. Don't be scared of a few scale weight pounds coming back on; that's part of the struggle. But it's not because you suddenly put on fat, it's because your body is refueling and rehydrating.