Paul and his companions, including the physician Luke, made their way from Miletus to Cos (Kos).
And when we had parted from them and set sail, we came by a straight course to Cos, and the next day to Rhodes, and from there to Patara. (Acts 21:1 ESV)
Cos is one of the islands belonging to a group of 12 called the Dodecanese. Patmos is also an island of this group. The Mycenaeans settled Cos in the 15th century B.C. In the centuries to follow the island came under the control of the Persians, the Greeks, and the Romans. In modern times the island has been under Turkish and Italian control, and German occupation. Since 1948 it has been part of Greece. My only visit to Cos was a brief stop en route from Patmos to Rhodes in 1984. Here is one of the photos I made.
In the time of Paul, Cos was noted as the birthplace of Hippocrates, the “father of medicine.” Hippocrates was associated with the Asclepium, ruins of which can still be seen. A Hellenistic gymnasium and some Roman ruins, including portions of a Roman road, may also be seen. There is also an archaeological museum.
Howard F. Vos describes the island with these words:
One of the most beautiful ports of the ancient world, Cos not doubt was most famous as a health resort. It was the site of the first school of scientific medicine and the sanctuary of Asclepius (Esculapius). The island had a healthful climate and hot ferrous and sulfurous springs, which the great Hippocrates (ca 460–377 b.c.), the father of medicine, first used to cure his patients. (The International Standard Bible Encyclopedia, revised.)
James Strahan, in the old Hasting’s Dictionary of the Apostolic Church, says,
It was renowned for its vines and looms, its literature and art, and above all for its temple of Æsculapius and school of medicine, which must have made it especially interesting to St. Luke.
According to Josephus, Herod the Great assisted the people of Cos with grain and other goods. (JW 1:424).
Two Other Good Sources:
Fant, C. E. and M. G. Reddish, A Guide to Biblical Sites in Greece and Turkey.
Wilson, Mark. Biblical Turkey: A Guide to Jewish and Christian Sites of Asia Minor. This book deals with Turkey, but devotes ½ page to Cos as a Sidetrip.