Demons can possess anyone who is not a Christian. (that is has repented of their sin, accepts Jesus as the son of God and asks Him to be Lord and Savior of their life). Once they are saved they are indwelled with the Holy Spirit.
At that point they can no longer be possessed (the Holy Spirit is not going to share His temple with demons). However they can be oppressed. If the enemy can lie to us enough and get us to believe something (such as we are ill) then he's got control of our minds. We are not possessed but we are oppressed. Subtle but important difference.
According to these exorcists, the biggest risk factor for demonic possession is the victim's previous involvement with the occult.
True. Don't open doors that you don't want used.
Additionally, exorcisms can become violent. It can take several grown men to hold down a possessed person.
Or one person full of the knowledge and the power of God. We have all authority over all the power of the enemy.
True, and an important distinction. Christians can still be demonized (simply having the Holy Spirit indwelling the temple of your body no more prevents demons from entering than having the glory of God in Solomon's Temple kept the Jews from populating it with idols. However, we cannot be totally possessed by the Enemy, being already the possession of Christ.
But that doesn't mean that demons cannot cause severe problems or that Christians should be under the belief that they cannot be attacked or even indwelt by the demonic if they do not take the authority Christ gave them to kick the demons out.
Makes sense.
The fact that so many people in our society have been baptized is also one explanation for the apparent lack of demonic activity in our age, at least in comparison to Biblical times.
But as our society becomes more anti-Christian, overtly demonic manifestations will increase.