Meet the loa loa, colloquially known as the african eye worm. When bitten by the deer fly, the vector of the loa loa, the worm will infest you with a condition called loa loa filariasis; the infestation of the parasitic worm. It then migrates through-out the tissue of the human body, occasionally crossing the tissues of the eye, where it is visible and clearly observable. It is not painful as such, and it does not degrade your vision in any way, but moving around the eyeball is often a painful and extremely uncomfortable, both physically and emotionally, sensory experience. The surgical means of getting it out can also be agonizing. Worms 20 centimeters of length have been found in human eyes.