Evil spirits in Christianity
Many Christians throughout history have believed in, and still believe in, the existence of evil spirits. This doctrine occurs across Orthodox Christianity, Catholism and Protestantism. They are mentioned many times in the Bible. Evil spirits are thought to enter a person and cause certain negative behaviors or diseases. In other cases, evil spirits can persecute or oppress a person. Certain holy people are believed to be able to remove evil spirits from a person; this process is sometimes referred to as exorcism.
Evil spirits are mentioned though out the Old and New Testaments.  Evil spirits can cause dumbness Mark 9:17-29 , hunched back Luke 13:11-13 , self harm Mark 5:2-13 , convulsions Mark 1:26 , seizures Matthew 17:15 and blindness Matthew 12:22 .
Mark describes a man that was possessed by a demon called Legion and he could not be restrained because Legion gave him strength to break chains. Day and night, he would cry out and cut himself with stones. Jesus cast out Legion into a nearby heard of swine, unnecessarily causing their deaths. Mark 5:2-13
"The faithful angels are a greater multitude than the fallen angels. For sin is contrary to the natural order. Now, what is opposed to the natural order occurs less frequently, or in fewer instances, than what accords with the natural order."
Belief in modern Christianity
In 2008 a study, using data from the 2005 Baylor Religion Survey, revealed that 47.7% of Americans believed in demons in absolute terms; 57.8% of the population said the same of Satan and 55.6% of Hell. In comparison only 11.5% decidedly denied the existence of demons. The number of absolute disbelief is even lower for Satan (9.5%) as well as for Hell (9.2%).
There is some debate as to if a true Christian can be inhabited by an evil spirit.
The origin of evil spirits is not explicitely discussed in the Bible. Some Christians believe they are fallen angels , or disembodied spirits from the descendent of angels and human women Genesis 6:1-2 .
Evil spirits are less common than in Biblical times
Evil spirits do not exist
There is no evidence that evil spirits exist. Since they cause behaviour changes and disease, they should be relatively easy to observe. However, no such phenomena have been reported by reliable witnesses. Modern medicine operates well without making reference to evil spirits. Many of the behaviours and illnesses thought to be caused by evil spirits have been found to be caused by illnesses or biological causes.
The apparent non-existence of evil spirits is an argument against a literal reading of the Bible.