Toaru Majutsu no Index - Volume 3, Chapter 3: Railgun. Level5.

Thousands of years ago, the great abilities of the three teachings of Buddhism, Daoism, and Demonism decided on a place for becoming immortal. Three thousand years ago, a Taoist was close to becoming immortal. Three hundred years ago, the Daqian Dynasty was established, and chaos is expected in another three hundred years. In this era, the path to immortality opens; a celestial swordsman freezes fourteen states with one sword, a heavenly Daoist maiden seeks eternal life, a naturally born Holy Son claims to be invincible, and a Buddhist Holy Monk saves all sentient beings.Gu Wen lives as a beggar, enters a household as a servant, witnessing the disordered world, drifting aimlessly.In a world filled with divine beings, he alone is a mortal.However, a lowly origin is not a disgrace; striving through the grand path, even as a common man and mundane son, he can still be the foremost in the world.“Mortal Gu Wen, my Dao is euqaling the heavens!”

The whole world is subjected to a highly dangerous standardised exam called the Global Examination. Answering questions by putting their lives at stake, they would live if they passed the test. Every month the exam will undergo reformation while, occasionally, it is random.

The Ning family finally found the sole granddaughter after she disappeared for ten years. Grandpa Ning was overjoyed! He embarked on the search for a grandson-in-law in River City. With a raise of her finger, Ning Dai pointed at the Mo family's disfigured Second Young Master, Mo Chen. Everyone thought she was crazy, but only Ning Dai knew that when she was kidnapped, Mo Chen risked everything he had—even with a broken leg and disfigured face—to rescue her from the devil's lair after her elder brother died tragically. Ning Dai returned to repay Mo Chen for saving her life, and also to exact revenge on her uncles for her elder brother's death. Everyone claimed that the Mo family fell from grace after Mo Chen's injuries while Second Young Master Mo Chen became paranoid and ruthless too. No one dared to get close to him. However, Ning Dai insisted on clinging to him. "Second Brother, your medicine won't taste bitter today because I'll give you a kiss to go with it." Mo Chen was about to lose control of his emotions. "Scram. I don't love you." Ning Dai gave him a cheeky smile, "Two kisses, then." Eventually, Mo Chen kissed Ning Dai's forehead shakily, his resolve shattered into a thousand pieces. After seeing how profound Mo Chen's love for her was, Ning Dai was convinced that he would once again stand up for her and stay by her side at the top of the world.

The story is about a man whose compulsive tendencies make it impossible for him to keep a job, and a young girl who skips school and loves insects. The two meet, come to support each other in reintegrating into society, and fall in love. There’s a problem, however: namely, the parasites in their heads.