Cancer

Cancer, known scientifically as a Malignant neoplasm, is a disease in which affected cells can display three of the following properties:
 * Uncontrollable cellular reproduction above normal rates (this forms a Malignant tumor);
 * Invasion and destruction of neighboring tissue; and sometimes
 * Metastasis, a state in which affected cells spread throughout the body through the blood.

Halo Universe
Cancer ceased to be common in the Human population in the 22nd century, though cases appeared from time to time as far as the 26th century. These could be treated easily, by this time however.

At some time during the Human-Covenant War, Michael Baird had this disease onboard the UNSC The Heart of Midlothian.