All the scribes who came down from Jerusalem were saying, "He (Jesus) is possessed by Beelzebul, and by the prince of demons he casts out demons."
The Bible, especially the New Testament, has many references to demons and evil spirits. One of the most disgusting accusations made against Jesus was that he was possessed by Satan (here called Beelzebul). Jesus was the opposite of Satan as he was sinless, holy and good.
Satan, demons and evil spirits are real and they always act in evil ways. The Gospels and the early Epistles mention them a lot because it seems Satan was trying hard to destroy all that Jesus came to do — but he failed. The effects these evil spirits or demons brought about in people were often like severe illnesses, possibly like epilepsy, or some violent reaction to the presence of Jesus. But the Bible always makes a difference between a real illness and the work of a demon.
In modern times demonic activity is just as strong as it was in Bible times. People sometimes even want to be possessed by a spirit for one reason or another — witchdoctors, clairvoyants and people using a ouija board etc. Christians must avoid such things as they are evil and never 'for fun'. Keep away from them. Christians sometimes worry in case they could become possessed by an evil spirit. The answer to that is 'No'. As a Christian, we are given the gift of the Holy Spirit and he will keep us safe.
Dear Lord, keep me safe from the devil and all his works. Amen.