Cancer

Cancer was a class of diseases among humans and other species. It was characterised by uncontrolled division of cells and the ability of these cells to invade other tissues. This unregulated growth was caused by damage to DNA, resulting in mutations to genes that controlled cell division. If untreated, cancers could kill. Cancer cells were even sometimes considered to be immortal. Due to this, there was a rumour spreading during Miracle Day that, since everything mortal became immortal, cancer cells became mortal, which Allen Shapiro took as a good enough excuse to start smoking again. (TV: End of the Road)