Enigma:

A visual novel with multiple endings set in a fantasy world. "Enigma is a mystery visual novel where you discover the secrets of Enigma, a worldwide epidemic, and a forest on a lonely island that bears its name."

The world is afflicted with a mysterious disease known as "The Enigma." Chester, infected with this disease, finds himself washed onto a lonely island, Carlyle. As he is being given a tour of the island by the girl who nursed him back to health, Chester finds himself feeling that something is off....