Chapter five, of Mary Shelley's Frankenstein, is filled with great fear and illness for Victor. At the opening of the chapter, Victor tells readers of his success at re-animating life. Horrified that his "son" is not the beautiful being he desired, Victor flees his flat in fear.
After his initial escape from his flat, Victor happens upon Clerval (his best friend). Clerval, knowing that Victor has not contacted his family for a very long time, suggests that he go to Ingolstadt to check in on him. It just happens that Victor is pacing the streets at the exact moment Clerval arrives.
After a short welcome, Victor brings Clerval back to his flat. Although he is fearful the Creature may still be there, Victor finds that the Creature has left. Victor begins running around the flat, and Clerval becomes increasingly confused. Upon asking about why Victor is acting in such a way, Victor yells at Clerval to not ask him about it and falls down ill.
Clerval, being the only one who could nurse Victor back to health, stays on as Victor's nurse. He attends to all of Victor's needs until Victor is healthy enough to return home.
Essentially, Victor's illness comes from many different things. First, the attention to his experimentation with re-animating life forced him to neglect his own health. Second, the scare he took upon the Creature coming to life impacted his mental health. Lastly, his cold and wet night on the streets could have attacked his already weak immune system. With everything he had faced and done, Victor was very ill and needed Clerval to care for him.
While Clerval did stay by his side and care for him, it was not until spring came that Victor's health really began to improve. True to Romantic fashion, nature proves to be powerful in the novel given its regenerative nature. The promise of spring and new life gave Victor the ability to look towards his own regeneration. (This is highlighted in the theme of nature verses nurture.)