The problem of illness
Trophimus was a travelling companion of Paul, first seen in Acts 20. In Acts 21 Trophimus is described as an Ephesian - he was likely converted during Paul's ministry described in Acts 19.
Trophimus was left behind in Miletus due to illness. Paul would have prayed for Trophimus, as he does elsewhere with positive effect, but Trophimus was not healed and his debilitating illness prevented him from fulfilling his ministry. What should the believer do when faced with illness?
Through Job's severe testing he remains steadfast in faith. Testing of faith produces steadfastness (James 5v11).
Epaphroditus, introduced in Phillipians had been severely ill (to the point of death), but God restored him to health to continue his ministry.
God's grace is sufficient.