Indeed he was ill, near to death. But God had mercy on him, and not only on him but on me also, lest I should have sorrow upon sorrow. (Philippians 2:27, ESV)Epaphroditus almost died. Whatever this illness was, it was one that nearly took his life.
Paul recognizes that the reason Epaphroditus is still alive is simply because of the mercy of God. God had mercy on Epaphroditus. God also had mercy on Paul, because if something would have happened to this faithful friend, brother, worker, soldier... Paul would have experienced sorrow topped with sorrow.