Why Lord Jagannath falls sick for 15 days

One thing to note about the Rath yatra is that just before it starts, Lord Jagannath falls ill for 15 days and stays in isolation. Devotees cannot visit the temple during that time as it is believed to be his resting phase. But, why does this happen? Afterall, he is a God. Well, there are many theories and legends that answer this question.