20. Microscopic Invaders
For just a second, consider what microscopic means. For those unaware, micro typically means small. Thus, the invasion would typically be something that would be so small that it is likely unable to be seen by the naked eye. Usually, this has to do with viruses or infections that can pass through the air. However, for this example, we are going to use gas instead. There are several gases that are clear and unable to be seen.
Thus, someone could breathe in a gas that has been released into a home, locker room, etc. Usually, our bodies fight off particles floating around that we could breathe in. Our nose hairs fight a lot of this, funny enough. On top of this, granulocytes, lymphocytes, and monocytes help as well. However, if someone decided to make a gas that could bypass these natural protectors, this man-made gas could create a real zombie apocalypse.