Illness-death model
= Illness-death model
{title2=$1\to2,\quad1\to3,\quad2\to3$}
An irreversible three-state illness-death model allows progression $1\to2$ and death $1\to3$ or $2\to3$, with state 3 absorbing. A constant-rate <transition intensity matrix> uses three free rates $a,b,c$ and rows $(-(a+b),a,b)$, $(0,-c,c)$ and $(0,0,0)$. The labels can represent disease severity rather than literal health versus illness.