Demons are true spiritual beings living among us and they always try to oppose the works of God. These demons were angels before in heaven and their leader is Lucifer (Satan) They tried to rebel against God and God had to push them down from heaven. When God sent them away from heaven they came down and became evil spirits. They began to influence humans on earth with their evils. When Jesus came to this earth he cast away demons from humans, which clearly shows they can affect humans.
Isaiah 14:12-15 says,
“How you have fallen from heaven, morning star, son of the dawn! You have been cast down to the earth, you who once made nations tremble. You said in your heart, ‘I will ascend to heaven; I will raise my throne above the stars of God; I will sit enthroned on the mount of assembly, on the utmost heights of Mount Zaphon. I will ascend above the tops of the clouds; I will make myself like the Most High.’ But you are brought down to the grave, to the depths of the pit.”
Demons do not produce offspring like we humans, the increase in numbers by initiating other humans to their evil works. When humans sin by sexual immorality, stealing or anything that is sin against God, they become a demon at that moment because they're no longer holy in the eyes of God. Humans do not commit the sins themselves, they're influenced by demons to commit sins.
As Christians we should always pray and ask God to help us so that demons will always run away from us. Anybody who is prayerful and with the spirit of God, demons don't go close to the person.