When we are feeling sick, think “how wonderful, what an opportunity, this karma has ripened; I have been waiting for this opportunity (to practice). In all my lifetimes, how much negative karma have I generated, how many sentient beings have I harmed?” Meditate on this suffering and think “May I substitute the suffering of all sentient beings. May I be a substitute to take on all the suffering of sentient beings!” We should really have this state of mind during our practice!