It is certainly likely that some diseases in antiquity were misdiagnosed as demon possession. However, even though we live in "the age of science", we probably go too far in assuming that there is nothing spiritual and everything is physical. Just because we can "diagnose" or "describe" something does not mean there is a physical cause--only that there are physical symptoms. The idea that spiritual realities can have a physical impact is certainly feasible. I understand that some "diseases" or "disorders" have no detectable physical cause--only physical symptoms. So, spiritual causes should not be ruled out.
It should also be noted that for many disorders that can be diagnosed, there are no real cures and certainly none that are instantaneous. However, Jesus was able to cure them and does so in an instant.
There are some instances of Jesus driving out demons that would be quite difficult to attribute to mere physical causes, indeed. The account in Mark is one such instance:
They came to the other side of the sea, into the country of the Gerasenes. 2 When He got out of the boat, immediately a man from the tombs with an unclean spirit met Him, 3 and he had his dwelling among the tombs. And no one was able to bind him anymore, even with a chain; 4 because he had often been bound with shackles and chains, and the chains had been torn apart by him and the shackles broken in pieces, and no one was strong enough to subdue him. 5 Constantly, night and day, he was screaming among the tombs and in the mountains, and gashing himself with stones.
6 Seeing Jesus from a distance, he ran up and bowed down before Him; 7 and shouting with a loud voice, he *said, “What business do we have with each other, Jesus, Son of the Most High God? I implore You by God, do not torment me!” 8 For He had been saying to him, “Come out of the man, you unclean spirit!” 9 And He was asking him, “What is your name?” And he *said to Him, “My name is Legion; for we are many.” 10 And he began to implore Him earnestly not to send them out of the country.
11 Now there was a large herd of swine feeding nearby on the mountain. 12 The demons implored Him, saying, “Send us into the swine so that we may enter them.” 13 Jesus gave them permission. And coming out, the unclean spirits entered the swine; and the herd rushed down the steep bank into the sea, about two thousand of them; and they were drowned in the sea.
14 Their herdsmen ran away and reported it in the city and in the country. And the people came to see what it was that had happened. 15 They came to Jesus and observed the man who had been demon-possessed sitting down, clothed and in his right mind, the very man who had had the “legion”; and they became frightened. 16 Those who had seen it described to them how it had happened to the demon-possessed man, and all about the swine. 17 And they began to implore Him to leave their region.
18 As He was getting into the boat, the man who had been demon-possessed was imploring Him that he might [c]accompany Him. 19 And He did not let him, but He said to him, “Go home to your people and report to them what great things the Lord has done for you, and how He had mercy on you.” 20 And he went away and began to proclaim in Decapolis what great things Jesus had done for him; and everyone was amazed. Mark 5:1-20 NASB
This particular man had supernatural strength, was self-destructive, lived among tombs, went about naked (from Luke's telling of the story) and seemed to be in torment. Seeing Jesus, he/they identify Him as the Son of God and beg Jesus not to torment him/them, recognizing Jesus' power over them. He/they also identify as having the name "Legion".
At the command of Jesus, the disorder disappears but reappears in a herd of pigs, which then take on self-destructive behavior themselves and kill themselves. However, the man, who, moments before, had this "disorder", is now completely free from it and actually becomes a missionary, telling about what Jesus did for him.
What "disorder" accurately describes this case? Multiple-personality disorder? Is this ever healed in an instant by modern medicine today? Does it cause people to go about naked and give them superhuman strength? Does it make them recognize the deity of Jesus and fear being tormented by Him? Does the healing turn them into missionaries?
The best explanation remains that the accounts of demon possession in the Scriptures refer to something real that is spiritual in nature, although there is, in fact, a physical impact that can be seen. Diagnosis does not necessitate that the cause is physical--only that physical symptoms can be observed. There very well may be instances of demonic influence in the world today that are identified and "diagnosed" but which are merely giving a name to a spiritual reality.
Modern medicine is a truly wonderful blessing for the treatment of physical illnesses, but physical remedies cannot address spiritual realities.