You may or may not rely on contact lenses in order to see every day. Or perhaps, you know someone who does. Alternatively, maybe you need prescription lenses and are considering contact lenses instead of glasses just so you don't look goofy with your glasses.
Either way, you may want to take really good care of how clean you keep them, because they're prone to being the living space for all kinds of microbes, and you can bet those microbes are going to be transferred to your eyeballs when you put them on.
A common infection can be caused by these microbes - Microbial Keratitis is the name for it and it's essentially an infection of the cornea that can result from improperly-cleaned contact lenses.
Some microbes can actually adapt to your body's defensive mechanisms, and even the anti-bacterial solutions you soak your lenses in. These microbes can be the most dangerous.
Eh... the convenience of contact lenses sounds nice, but I think I'll stick to glasses.