Katniss sees Rue pointing to something above Katniss's head: a tracker jacker nest!
What are tracker jackers? Why, mutant killer wasps engineered by the Capitol, of course. A few stings make you hallucinate like crazy; more than a few stings and you're dead. Even if you run, these things will track you down.
Katniss realizes she can cut the branch holding the nest and drop it on the pack of Career Tributes. During the nightly anthem, she climbs up to the limb with the nest and does just that.
Before she can get the branch totally sawed off, though, the anthem ends. She decides to finish the branch off, and drop the nest, in the morning.
Returning back to her spot in the tree, she sees a pot with a silver parachute. A present from a sponsor! At last, Haymitch has come through. It's a good gift: high-tech ointment for her burn. She spreads it on her calf and hands, and they magically improve.
Katniss whispers Rue's name to give her a heads up about the nest. They lock eyes, and Katniss holds up her knife. Rue nods and disappears, leaping from tree to tree. So that's why she got such a high score in training.
Dawn breaks, and Katniss saws off the remaining branch. The nest crashes to the ground, and immediately Katniss gets three stings. She takes out the stingers.
On the ground, there is chaos. Most of the Tributes run for the lake, but the girl Glimmer gets stung and goes berserk. Eventually she falls to the ground, twitching hysterically.
With everyone cleared out, Katniss runs in the opposite direction of the lake. She feels the effects of the stingers: swelling and pain. She still has enough sense, though, to grab Glimmer's bow and arrows.
Struggling with Glimmer's body for the bow, Katniss starts hallucinating big time. The Career Tributes return, and she sees Peeta. He is shimmering. He screams at her to run – so she does.
More hallucinations: ants crawling all over Katniss. She trips and falls into a small pit and can only think: "Peeta Mellark just saved my life" (14.31). Then everything goes black.