All organisms on earth are made from the same basic part, mostly proteins. Proteins are the building blocks of life and can have billions of different shapes — you can imagine them as 3D puzzle pieces. There are billions of different puzzle pieces your enemies can use to construct their bodies. Why is this important? Because proteins are in a way the "language" of the micro-world. Cells don't have eyes or ears, so to tell friend from foe, they have to touch them and recognize if their protein is part of a friend or part of an enemy. Recognizing means that cells have countless tiny devices called receptors that are able to connect with a specific protein puzzle piece. So your cells have tiny puzzle pieces on their outsides that are able to click together, or recognize other protein puzzle pieces. When a cell connects together with a protein and recognizes it as "enemy", it knows that it has to attack. Only if your cells can make this distinction between friend and foe is your immune system able to fight an invader. But since there are billions of possible protein puzzle pieces, this means there are billions of possible enemy puzzle pieces. This is also one of the reasons we still have to deal with diseases like the flu each year, the influenza virus mutates very rapidly and so the proteins that make up its hull constantly change a tiny bit. The soldiers of your innate immune system have a large number of the puzzle pieces for common bacteria and viruses memorized, that's why they are your all-purpose weapons.