Gonna eat soon? I hope not, because we're going to talk about pus. Although mostly it's white, it can be yellow, red, green, and blue. The color depends on the kind of stuff that formed it. Let's find out about the ecology of pus.
The moment anyone hits puberty, it becomes apparent to them that their body is just a comfortable staging ground for vast numbers of microorganisms. Some of those microorganisms help them digest their food and can even do battle with less-pleasant organisms. Some of the organisms that move in do not play nice, introducing people to an unpleasant little thing called infection and a equally unpleasant little thing called pus. To be fair, we are mostly to blame for pus, as it's mostly made up of the corpses of our fallen white blood cells. But there's nastier stuff in there as well.
The garden variety white and yellow pus is the result of the most common infectors - Streptococcus pyogenes and Staphylococcus aureus. Pyogenes is behind everything from minor infections to strep throat to scarlet fever. It can destroy red blood cells, and can even cause toxic shock syndrome that develops into necrotizing fasciitis. Aureus, which is yellow and can bring that color to pus, it responsible for those famous staph infections - because they're already on our skin most of the time. They're just waiting for a chance to get in.
Green pus is formed by Corynebacterium diphtheriae. These bacteria come at you through the nose, and produce toxins that go through the membranes on your nose to various parts of your respiratory system and beyond. This is the bacteria behind diptheria, and it can cause inflammation of the heart and damage to the nerves if it's not treated with appropriate antibiotics.
And finally we have blue pus, which is pretty rare, but which you should look for if you have an infected burn. Burns are particularly suitable habitats for Pseudomonas aeruginosa, which just goes for straight-up blood poisoning in its attempt to kill you. This is such an interesting bacteria that it got sent to space. What happened? It adapted so well to the microgravity that it formed a special canopy structure when it formed biofilms. This has got to be the one that took down the aliens in War of the Worlds. (By the way, you can also often get it from hot tubs. Enjoy your soak.)