I was around 8 or 7 at the time I got Halo: Combat Evolved, so once I got up to the level 343 Guilty Spark, I continued along through the swamp, being a tad spooked by the large and open, but foggy area around me, but once I jumped up onto the fallen tree that allowed for a way to the inner facility and saw the yellow dots on my radar, but looking directly at their outlines in the distance and knowing that something wasn't right, I continued onwards and originally thought that there were some stray marines fighting the remains of the covenant when I started to hear gunfire from assault rifles... I was wrong.
Once I reached the place where The Flood were ready to pounce on me, I watched the beginning scene and felt intrigued to be fighting a new enemy after the scene had finished, but after a lot of green blood had been poured over the entire room, I recall being quite surprised by the fact that flood-infected elites were jumping at me, but I persisted nevertheless.
As I continued to fight The Flood in the underground facility, fear started to creep up on me as the dark rooms ahead and unnerving ambience eventually got the better of me, so once I couldn't go any further, I went back to a sunny and moderately happy level of the game for a while until I gained the courage to take on The Flood again.
Determined to finish what I started, I loaded the level 343 Guilty Spark and battled my way back to where the introduction to The Flood was, and once it was time to take on the scary hallways with Flood jumping out at you or suddenly attacking you from behind, I started to call them ugly and hideous freaks that were very weak, and just blatently insulting them in order to make myself feel less scared, and it worked.
Once I reached The Library, I originally thought it was going to be just like the last level, but I was quite mistaken because of the fact that The Library was a much more illuminated and open level than the previous one, and that made The Flood seem less scary to me, but I do admit that once I completed The Library and began fighting covenant again, I felt relieved and proud to have achieved victory over the horrors of the level 343 Guilty Spark.
So there you go, that's my experience with The Flood on Halo: Combat Evolved.